Leaving too many decisions to an AI regent will make the game unplayable for some.
When i got into a regency I typically acted as though I were playing the regent (I usually did my best to avoid a minor inheriting anyway...). Sometimes I would give him land and money or give him a princess to marry, and generally tried to act according to his interests and personality. When my king came to power I would go about reversing those things if I felt like it. But I think that was only fun for me because I wanted it. If I hadn't any control over the situation I would've been annoyed as hell.
After all, the regent still has to deal with the king's own family, after all the regent has to deal with a council that is potentially full of loyalists to the king, after all the regent has to deal with a court and kingdom that expects him to defend the king's interests. Using a regency to enrich oneself is corruption, and even the people of that time would see that.
So on the one hand regents should probably attempt to launder some money or draw up land grants to themselves and their friends, and force the kingdom into wars they want to wage... but on the other hand regents should largely do as the king would normally do, do as everyone around them expects them to do. CK2 basically accomplished this.
The game still has to be fun, and part of the fun is being able to actually play it.