I wish for an improved peasant rebellion. The current version is simply dull and pointless - if you are a powerful lord, you simply crush them on your way to your next major war, if you are a minor count, all you can do is sit and dawdle till the peasants finally get bored of sacking your castle. Hell, you cannot even call your liege for help, isn't he meant to protect you for god's sake?
I think some mods already improved minor rebellions, in a way that they give a penalty to taxation and prevent you from raising your levy in that county, instead of creating a rebel army. But I consider this only step one. There should be also major rebellions that spread like wildfire and endanger whole duchies.
My idea would be as follows:
1) Minor rebellion. Can be stopped by your marshal (randomly) or via event.
2) Infliction. If the rebel's demands are not met there should be a chance that the peasants in neighbouring provinces get drawn into the conflict and rebel as well.
3) Once a whole duchy rebels (or something like that) a rebel army should spawns that behaves like an invading mercenary company. That would allow you to call your allies into the conflict.
Each time a rebellion is started or it gets worse, you should have a chance to call your liege for protection. This should cost you a lot of prestige, though. Perhaps the options could be even determined by your traits - a wroth character won't be able to deal peacefully with the peasants, while an ambitious character won't call for help before the invasion starts.