What I'd like to see in the future are:
I. Reworked Emperor/Empire mechanics: HRE too strong, Byzantines(?), more possible empires
IA. Holy Roman Empire: it's too powerful and stable, it's been stated before. In the state it's in now I'm fairly sure it's impossible to fully eliminate. How do we fix this?
IA1. Reworked Crown Authority for Empires (see IB)
IA2. HR Emperor is elected by all vassals with duchy/king titles, who are the "electors"
IA3. HR Emperor MUST be approved and crowned by the Pope, otherwise a more acceptable candidate is elected
IA4. Elector AI is tweaked to vote more in line with own interests (From what I've seen there's significant bias in favor of previous Emperor's heir for some reason)
IA5. HRE vassals can create their own kingdom/duchy titles (I disagreed with this removal in 1.05, as it was a move toward more centralization which is not what HRE needs) (also see IB)
IA6. There is a set Imperial Capital province that exchanges hands whenever new Emperor is elected; if HRE is dissolved without a sitting Emperor the leading baron becomes count of the province
IA7. HR Emperor has option to abdicate (see also IA8)
IA8. HR Emperor can at any time hold vote to dissolve HRE; HRE vassals have plot option "Dissolve HRE" and can hold a vote if they gain enough plot power (Pope has lots of influence in plot power for this as well)
IA9. Electors have plot option "Depose HR Emperor" which dethrones current Emperor and holds a new election; ambition/plot "Become HR Emperor" also exists
IA10. If a single HRE vassal gains 51% of the de jure HRE and one of the King titles under it, he can choose to "Dissolve HRE" by declaring his own independence; this completely dissolves the institution without a vote and makes all existing HRE vassals independent as well
IA11. This part is rather vague: give the HR Emperor rigid piety requirements or be automatically excomm'd by the Pope; make HR Emperor-Pope opinion relations scale more sharply based on his piety
IB. Imperial Crown Authority
IB1. Imperial CA (ICA) would be different from Kingdom CA, ranging from Minimal, Low, Medium, High; ICA influences (read: caps) the tax "sliders" (it is made a single entity rather than feudal/city/church and replaces the vassal levy slider entirely
IB2. Minimal ICA: Emperor cannot revoke titles, cannot collect taxes from vassals, cannot raise any troops from (count+) vassal levies without Call to Arms (see II), is elected by electors/approved by Pope (latter for HRE), vassals can fight each other freely, vassals can create own de jure titles (ie kingdoms/duchies) but Emperor cannot
IB3. Low ICA: Emperor cannot revoke titles, can collect limited taxes from vassals, can skim off the top of vassal levies without CtA, is elected by electors/approved by Pope (latter for HRE), vassals can fight each other freely, vassals and Emperor can create own de jure titles (but Emperor must give them away or face severe opinion penalties)
IB4. Medium ICA: Emperor can revoke titles, can collect reasonable taxes from vassals, can skim reasonable troops from vassal levies without CtA, is elected by electors/approved by Pope (latter for HRE), vassals can fight each other but must seek Emperor's approval first (minus plots), vassals and Emperor can create own de jure titles (Emperor does not face penalties for keeping them)
IB5. High ICA: Emperor can revoke titles, can collect any amount of taxes from vassals, can skim highest amount of troops from vassal levies without CtA, title becomes inherited by children (but HRE must still be approved by Pope), vassal cannot fight each other, vassals cannot create own de jure titles, Emperor alone can create and give away titles
Note: Elections do not apply to the Byzantine Empire, which as far as I know is inherited (or at least elected from within the dynasty)
IC. Byzantine Empire: I'm not very familiar with Byzantine Empire at all (bare bones, basically) but I still feel Byz is too stable, with not enough internal strife that causes borders to shrink (I mean to say I do see lots of civil wars/rebellions but their borders only ever expand, their neighbors never seem to take advantage of it and vassals rarely become independent)
IC1. Maybe some AI tweaks to make Byz vassals/courtiers more self-serving? I'd like to see Emperors other Doukas sometimes
IC2. I think I've read in places that the neighbors of Byz (ie Seljuks especially) need AI tweaks to make them more aggressive/effective against Byz. I don't know.
ID. Other Empires: Other empires should be able to form somehow, either through the Pope or a combination of cultures (this suggestion will be inherently ahistorical)
ID1. Cultural Laws: instead of having the culture of the ruler/conquering nation overwriting all the others automatically, give several options that can help represent the multiculturalism that comes with large empires
ID1a. Dominant Culture: The culture of the ruler or dominant kingdom/original conquerer within the empire takes preference and gradually converts the other provinces in the empire; causes noticeable opinion penalties with vassals/courtiers of different culture and severe revolt risk in provinces of other cultures
ID1b. Multiculturalism: Cultures within the empire do not spread across provinces and remain static
ID1c. Pan-Cultural Identity: A new culture forms from existing ones (examples: British from English/Welsh/Scottish/Irish, Pan-Germanic/Teutonic/Gothic(?) from German/Bohemian/Dutch/Austrian/Burgundian, Pan-Slavic from Russian/Polish/Hungarian/Bulgarian/etc., French from Frankish/Occitan/Burgundian, Pan-Italian/Latin(?) from Italian/Sicilian/Lombard, Spanish/Iberian from Catalan/Basque/Castillian/Portugeuse, Scandinavian from Norse/Danish/Swedish/Finnish/Lappish and others) and converts provinces; causes opinion/peasant happiness bonuses in vassals/courtiers/subject provinces of same/root cultures but the opposite in those of different cultures
ID2. Kingdom titles should be split from the Emperor: Emperor can keep king title of his native culture/capital location but must give away the others or face severe opinion penalties
ID3. Inheritance laws are restricted based on Cultural Laws: non-elective is restricted in Multiculturalism but not in the others, though non-elective law causes major opinion/revolt risk penalties when used
ID4. Ideas or these kinds of empires
ID4a. Kingdom of Great Britain (de jure England, Wales, Scotland, and Ireland)
ID4b. French/Frankish Empire (de jure France, Brittany, Aquitaine, Burgundy, Savoy, Lombardy, Flanders/Netherlands, Aragon, Navarra)
ID4c. Kingdom of Scandinavia (de jure Norway, Sweden, Denmark, Finland, Pommerania(?), Courland(?), Livonia(?))
ID4d. Spanish/Iberian Empire (de jure Aragon, Navarra, Leon, Castille, Galicia, Portugal, Andalusia, Murcia)
ID4e. Kingdom of Italy (de jure Savoy, Lombardy, Venezia, Tuscany, Sicily, Burgundy(?), Rome(optional))
ID4f. Russian Empire (de jure Rus, Courland/Livonia, Ukraine, Finland, Georgia)
ID4g. German Empire (de jure Germany, Flander/Netherlands, Bohemia, Austria, Burgundy, Savoy, Lombardy, Prussia, Poland, Denmark) (NOT THE HRE)
ID4h. Slavic Empire (de jure Poland, Hungary, Prussia, Courland/Livonia, Bulgaria, Bohemia)
ID4i. Turkish/Anatolian Empire/Sultanate (de jure Byzantium/Turkey, Bulgaria, Greece, Armenia, Georgia and Muslim culture/religion)
ID5. Empires should only be able to be declared if 90% of de jure land/titles are under control of one realm (or papal involvement or cultural requirement, I don't know)
Note: Again this is meant to be very ahistorical. The idea of pan-culturalism is basically modern-ish nationalism waaaaay ahead of its time, much less unified empires in places like Germany, Scandinavia, Britain, and Russia.
II. Call to Arms: I've always been annoyed that when I'm locked in a desperate struggle with another large kingdom my vassals don't do crap with the manpower they have and I can never get, often fighting their own little private wars instead (this is something of a call-back to CK1, I believe)
IIA. In times of war or rebellion an Emperor/King/Duke/Count can issue a "Call to Arms" to all his vassals, mandating that they raise their full strength and join his banner
IIB. Call to Arms can be accepted or refused by each vassal: an acceptance raises all his levies and places them under their liege's direct control; a refusal causes a major but not catastrophic relations hit with liege
IIC. Vassals have the option to appeal to liege to return their levies or pay for upkeep (latter as per usual), the former in case of a rebellion of their own
IIC1. If liege refuses the appeal vassal can order his levies to return to his control, causing a significant relations hit with liege, or grin and bear it
IID. AI would need to be tweaked so they don't immediately CtA for small rebellions or tiny peasant revolts
IIE. Emperors (especially the HR Emperor) should be heavily restricted/penalized/whatever from making a full CtA, so as to balance an Emperor's power with the ability for empire vassals to rebel and gain independence.
Again this is a detailed wish list and I don't expect to see any of this very soon, if at all. But if I do then <3 paradox
Clearly I put too much thought into this. EDIT: Wow this post was way longer and way more Wall-of-Text-tier than I had expected