Here are some of my suggestions that I would really like (some of which I haven't seen...)
-The obvious unlock everything suggestion...
-Papal election...The present "create-a-pope" method needs to be changed. We need to have people vying to put their men in Rome, and bribing, and intrigue!
-Friends and rivals...This is important so it's easier to role play and think about why you would do this action versus another. Maybe add "Your Friend" when you hover over the person. And of course, events that would make people become friends or rivals. Also add the possibility of friends who aren't really your friends (if they have high intrigue, they should be able to plot to "become friends", or "become influential upon...") More on this in a sec.
-There should be some sort of travel time between England and Jerusalem, if one is crusading. Make it so they don't get back right away, and that there is a regent in charge (and you should be able to appoint them too....). You may be able to come back and find England in civil war (IE, the FOW shouldn't update nation changes right away). Make it so FOW is based off what YOU personally, the person you are playing as is aware of. Intrigue should have an impact on this. Also regarding this, no pregnancies when the husband is away!
-A minor feature, you should be able to see titles held in their lifetime for dead characters rather than just titles held at the time of their death. Maybe put a sword or something in the corner of the flag to symbolize that they lost the title....
-Have distant branches of your family break off, cadet branches....No explanation needed
-Traits should be both good and bad. The traits you get when you become an adult should be of some use other than a boost in abilities. In regards to good and bad traits, there should be some of both. Kind people shouldn't be likely to do certain things, and should be more apt to let people free from jail. Envious people should be more likely to fabricate claims, and Just people shouldn't be likely to fabricate claims. People who are charitable should lose some of their tax, and people who are arbitrary should be more likely to steal (taxes), or be more likely and able to declare war without real cause. Traits that don't agree with certain people should make them not want to do stuff for the person, they should not accept marriages, or council invitations, and should be more unruly to them. Possessed should have some fun events too....And should also make people be repulsed by you.
-There should be miscarriages and stillborns. I think that it's an important part of the time period and should be implemented. In return, there should be more incidences of child death, and there should be an increased chance of everyone getting ill, and more likelyhood of them dieing because of it.
-When searching character screens, or marriage screens, your should be able to see if a given person is in line for a title, outside of the heir. You should also be able to see a quick way to check the succession laws there.
-There should be some variation to opinion. Maybe there should be more events to change them up, but there should also be some just random variation....Maybe under the category of "First impression".
-You shouldn't be able to see a given persons traits or skills. When you meet a person, you don't know "Ah, this man's deceitful", you figure it out of your own. This is where trusting can come into play. You may trust him, so you seem to see an inflated view of how good he is at anything. There should be some mechanic to know whether you've met him or not, so you can begin to form an opinion (other than that if you know he's a different religon or culture, you may have reason ((though not justly)) to dislike him). Your diplomacy skill can increase the odds of seeing his abilities (or seeing the wrong ones...), and so can intrigue. A good spymaster should be able to see him (and scheme or build spy network should uncover some more). In effect, make it so that while a character has a set rating, it isn't necessarily shown to the user. The same should be done to himself. A proud man would be impressed with himself and have a better view of himself, etc. You should also be able to see definitively what your opinion of others is, but their opinion of you should be accurate or inaccurate, based on your skills. With this, people should look at themselves in THEIR opinion when deciding whether they should object to their liege about the council. Make skills important.
-Have regents possibly take over. As I discussed, you shouldn't be able to see for sure what is happening in your home realm. There should be possibility for turmoil while you're gone. Regents should be more likely to usurp if they are regenting (if that's a word) a child, or an old incapable man. More on this in a sec....
-Incapability and those related traits should actually have an impact. Deceitful people should try to take over more if their ruler is such. There should be more chances of assassination or plots against them so the son can take over for dear old dad who went off his rocker. There should be peaceful usurpations in these cases, because someone who is crazy should be able to be taken down easily.
I hope my suggestions were helpful (although I know that they require major overhauls and would likely never be put in...)
EDIT: Sorry, I have a bit more...
Make truces matter more....And tone them done a bit while you're at it.