MOST IMPORTANT:
*More events, more plots, more ambitions.
*Additional emphasis on friends and rivals. Duels. Gregarious characters should have 2-3 friends, normal people should have 1-2, and shy should have 0-1. Friends shouldn't be rarer than Charizard cards. Controlling a title, within which a character independent of the title-holder controls lands, should make the two characters rivals. Otherwise, ambitious characters should have a high probability of having some other rival. It should be uncommon but not rare. Friends and rivals should be clearly marked and obvious in some way, not hidden unless you check relationship status nuances.
*Fleshed out Tournament. You can only do it once in your life, but there are hardly any events attached other than "So-and-So" won, or "So-and-So" died or "Do I attend my liege's tournament?". Same with Ramadan, and Crusades, IIRC - very few events. There should be entire event chains associated with these once in a lifetime events, *at least* on par with the Hajj event chain.
*Independent Realms should have a Faction System for (at least some) war declarations, so I can conspire with other rulers in an alliance to attack somebody else, rather than declaring war and hoping, after the fact, that my allies will join me - with no possibility of non-relatives joining, despite the fact that some neighbors of the realm I'm attacking may very much like to aid in cutting their neighbor down to size. Seriously? Nobody talked their war plans over with their allies in the feudal age, to work out who was willing to join before the war?
*My levies should be raised with a general leading them. It's ridiculous that they have no generals unless I choose to put one in charge, or if the army is raised from my personal demesne. As if anybody would just send their levies to me without ANYBODY in charge.
*Embark All button - with naval warfare. Otherwise, automated boats. Boats without naval warfare is pointless micromanagement - when I tell my army to go from Point A to Point B, calculate the path that will take the least time. If it's over water, send them over water and charge me for the generated "boats" as they use them.
*If I am the de jure liege of a count, while another person is the de facto liege, and the count loves me and hates his de facto liege - he should have the option of asking me to accept a transfer of allegiance to myself, giving me de facto control of the county, and the former liege a strong claim against it.
*Overseas territories won by Empires, in Crusades or Holy Wars, should result in an independent state - ruled by a male of the winning dynasty who is not 1st or 2nd in line to inherit anything. The dynastic alliance with an Empire should be sufficient to give the territory a reasonable chance of survival.
*Declared wars should be able to attach more war goals, including the option to take one/some of the loser's minor children into the guardianship of the victor's choice.
*Create rally points, so when I raise troops they'll automatically go there - like in Victoria 2.
*Chronicle of your Dynasty - a separate file in my Saved Games folder, where the game records the major events of my ruler's life - marriages, births, and deaths of my close family (preferably, "close family" would be a modifiable setting). The war dates and peace deals, the inheritances, the major events of distant relatives - when they are declared King or Emperor, or win a war against a major power. Etc.
*Rework the Dynasty- and Realm- trees to be more user friendly. Dynasty - the filter system helped a lot but a drop-down menu or filters to let you see all the past and current landholders without the riff-raff would be nice. A filter to filter out the first generation children to other dynasties - I don't need to see them. Scroll bars rather than clicking and holding. Realm: Scroll bar. Search bar, so I can start typing in the name of a title and the other title-holders will melt away as their primary title names stop being possibilities - searching through it for a large kingdom, whether checking up on a fellow vassal or one of your vassals, is nearly impossible, despite being otherwise informative and awesome as hell.
*Dynastic opinions. When my close relatives intermarry another dynasty, it produces temporary alliances. Keeping these ties going for decade/centuries should result in members of our dynasties feeling more kindred. On the other hand, constantly declaring war and usurping titles from a dynasty, should make all members of their dynasty shudder in rage at the mention of my dynasty's name, not just the single king I'm attacking. There should be events where a member of the close dyasty and I meet in the market and have a talk, increasing our personal opinions, or we happen to be crusading together and our common experience strengthens our family ties more. Or one of my dynasty members sent to scheme in the capital meets a rival dynasty member who has married my liege's unlanded son. She calls her guards and, though the combat is fierce, the player receives notification that his spymaster is slain. Etc.
MAJOR CHANGES:
*Playable Pagans, with their own unique gameplay.
*Improved Papacy, House of Cardinals, etc. The Pope should be more active in the world - and particularly Italy - than just being the name attached to Crusades.
*Judaism - if the culture of Greece can change to German because they won a Crusade for it, I should be able to land and play a Jew, with their own mechanics.
*Cadet Dynasties - my sixth cousin twice removed, though I'm sure quite lovely, is hardly somebody I consider family. It would also help keep the muslim decadence system sane.
*Asia!
*Stronger Ethiopia and Pagans, or events/decisions they can enact which will make their neighbors less willing to kill them - like a tribute payment to earn a 10-year truce.
*Fleshed out Religious Heretics
MORE MINOR:
*A penalizing trait for characters born to lowborn parents, and the AI should strive to marry into highborn families.
*There should be an alert for increasing decadence. Hovering over it should give a list of names starting with who's giving you the greatest amount, clicking should bring you down the list, to the Characters' Character screens. Perhaps there can be a number on a corner of the alert showing how much you are currently gaining per month/year, so you can see any growth at a glance.
*The claimants screen in the ledger should clearly depict whether a character is of my dynasty
*Christian pilgrimages
*More regency events/options
*Search character lists by traits.
*End all plots button.
*Button to invite everyone (willing) to join my plot.
*Search by Character Relations. Right now, I can search Character lists by Everybody's Opinion of Me, or Everybody's Opinion of a Third Party, but I know of no way to search for My Opinion of Everybody. And it hurts my ability to roleplay.
*Ransom Anybody, assuming you have enough gold and the opinion modifiers work out
*The Ransomed Prisoner should have the option to come to your court rather than their original home, if they are unmarried, unlanded, and feature ambitious traits.
*Additional minor war casus bellis, for prestige, indemnifications, and minor errata - like freeing a prisoner, or imprisoning somebody caught plotting against you from outside your realm.
*Declaring war for something minor should give your vassals a minor opinion penalty, if they're Content, or an opinion bonus if they are Martially-inclined, and possibly give you traits - like Just or Arbitrary. So a Player-Controlled King might not be interested in fighting a Count to release a dynasty member, but if he can choose an Event option to get the Just trait from it, he might be more inclined to do so. Any wars which don't result in blobbing are good wars to implement/support.
*War exhaustion should ramp up exponentially as wars are prolonged, or if more wars are declared before war exhaustion clears out.
*Imperial Crown Authority should only affect duchies outside a created kingdom. Created kingdoms should be able to set their own crown authority, which should take precedence over imperial crown authority. This reflects the level of stability/instability within various regions of the Empire - just because the Empire as a whole is stable doesn't mean that it doesn't have unstable elements where things might be more autonomous.
*The higher Crown Authorities should unlock at different tech levels. Give the AI more of a reason to build those universities everywhere quicker, and the player more of a reason to choose the Learning education trait.
*I should be able to intervene in my vassals' wars - even if it gives me a tyranny hit, I should be able to have the option.
*My character should be able to send explorers out to search for holy relics, or to purchase other treasures, which can be added to my Vault - which, ideally, would be a button I can click to see a visual depiction of my collections (more artistic DLC!). Collecting Them All can be an activity for anybody, but especially for people who just want to stay Counts. The treasures I collect would accumulate over the generations and would give opinion modifiers based on whether people were envious of the collection, or they admired my family's piety, or whatever. Rather than every character starting out with a -12 opinion modifier, having a large collection of relics would show that you come from an esteemed family which means business, and your vassals would back off, decreasing the short reign penalty.
*When a message pops up saying that somebody is the target of a plot, I should be able to click from that notification and be able to see the target, so I can tell without going to the Plots interface whether Count So-and-So being plotted against is a dynasty member or just married to my dynasty or what. Because I don't care if somebody's plotting to kill somebody who married into my family, but I want them to stop if it's my dynasty they're killing. And the game could make it easier to figure that out.
*Vassals that would prefer to be under an invading liege (same religion while current liege does not, for example) when their primary title is declared for in a foreign war should be able to offer to join the invading kingdom in the war.
*Spend some gold to tear down a holding and all its improvements, so I can convert it to a different holding. Maybe cause an increased revolt risk as a consequence. Useful for those who want to optimize their counties in a certain way. But more fun to use as total warfare - holy warring for duchies during a succession crisis, knowing that they'll be taken back eventually when the crisis ends, so burning everything to the ground and setting your opponents back by decades. Delicious.
*Institute the ERE mechanics mentioned in previous pages
*Make battles count for more war score
*When telling the court chaplain to convert, switch automatically to the religious mapmode.
*Deadlier, more contagious plagues. Vanilla plagues do next-to-nothing, unless there's special ones that pop up after 1250 - which is about the farthest I usually get in my playing. It would be nice if there was a once-a-century purging plague.
*Close family, like nephews and cousins and such, should not be subject to the "Would be hostage to foreign power" guardian mechanic if both are independent rulers. Why can you trust a random baron just because somewhere down the line he swears fealty to you, but you can't trust your cousin who's your neighboring king?
*For the outliner, If an army is moving there is a green arrow. Crossed swords for fighting and buildings for seige would be nice.
*Default council positions. Instead of manually telling every councillor what to do when one of them dies or is replaced, it would be nice if I could tell them what to do once, save the setting, and then new ones will automatically do that job until I either turn off the setting, change the setting, or tell them to do something else.
*Rebels should have a purpose, like in EU3, rather than just sitting there. Independence for wrong culture provinces, conversion of neighboring provinces for wrong religion provinces, killing the local liege if an event's results made them restless, etc.
*More uses for unlanded courtiers and dynasty members. Instead of sitting around in court, doing nothing, heirs and spares of my dynasty should be placed into a common pool in the Military tab - visually similar to the muslim decadence tracker in the Religions tab. There can be tabs for whether you're a child of kings, dukes, counts, barons, or courtiers - after all, we'd probably want to honor the highest ranking ones more than others. Then the head of the dynasty can call on these loafers to be generals, men-at-arms, bannermen, ladies-in-waiting, or what-have-you. A bit of flavor, more possible events, and a chance to broaden my too-narrow general pool.
*More cultural melding. IIRC: Saxon + Norman = English. It would be nice if others were included, whether historical or fantasy.
*Assassination for gold attempts should not be instantaneous - it should take at least a few weeks for the assassins to get their money, get to the location, make a plan, and enact it. They should offer you a choice of plans and, depending on the victim's traits, some of them should be more likely (though not guaranteed) to succeed. For example, okaying the poisoning of his cup may increase the chance of assassinating a drunkard but okaying a plan to throw him off a wall would decrease the chances of success - loosened muscles and all that. Anything to give the player more of a reason to look at his character sheet and think of the NPC as an actual character with weaknesses rather than just a name on a succession chart.
*Layered de jure territories - a duchy should be capable of being de jure part of numerous kingdoms/empires, any of whom would have de jure claims on it. Just because Denmark holds some English duchies for 100 years, England should still be able to press de jure claims. And if France then takes them for 100 years, Denmark should also still be able to press their de jure claims. They have a long history there, even if it's been hundreds of years since they controlled it - their claim doesn't just disappear because "it's been a while"
*Slavery
*Trans-Saharan Trade. Trade. Right now, counties are only more desirable because of their location (coast for port income and faster levy travel in their boats = good) and max number of holdings. Trade centers would do a lot to make otherwise passable counties turn into irresistible gems. Ideally, centers would be dynamic and buildable or destructible by anybody, so the game isn't railroaded one way, but with duchies/counties featuring mostly fixed income potential, so more trade center construction = diluted profit and the destruction of all a region's trade centers = wasted earning potential. Wars, events, and such would affect a region's income potential
*Make councillors more effective the longer they stay in their position. Alternatively, attributes can be be medium, on average, when a child becomes an adult, strengthening to become more powerful as the character is a councillor or ruler for longer periods of time, and then decreasing in old age. It can be an automatic check once per year, as is done for education currently, but for councillors and other special positions. This way, there is actually a good reason to desire a councillor position beyond a +1? attribute increase. Events giving and taking attributes should also be much more frequent, rather than getting only a +1 to +4 total, typically, on any particular character throughout an entire lifespan.