Quote Originally Posted by TinCow
FYI: The reason you cannot pick your faction heir is because of the marriage alliance function. If you could change the heir anytime, you could simply keep picking unmarried men so that you could establish many marriage alliances. In order to keep marriage alliances significant, they need to be truly rare and special. It would have been far better if they had simply limited the number of marriage alliances in some other way, since I also hate not being able to pick my heir, but I suspect there were engine limitations.
If this is the only reason for not being able to change the heir, there is a simple solution: when you change the heir, alliance made by his marriage breaks.