There are a few small changes I would do, mostly buffing the AI:

- give AI loyalty bonus for all unit leaders to prevent too many rebellions
- give AI loyalty bonus for provinces, same reason as above
- make sure AI has easier time teching up and building more advanced units (I already mod out peasants in my games)
- change the behaviour of AI generals suicide attacking my lines

All the points above should be fairly easy to implement, too bad we can't mod them in.