I agree with the OP suggestion that a game mechanic that allows for disposing a regent from a keep would be helpful, yet I don't believe that it would be good for the game.
I had a similar experience where I wanted a character to die and waited 40-50 some years for her to do so. What caused me to want her death? She became infertile after an event and nearly killed off my keeps bloodline that I use for creating sagewights.
If it were possible to kill off or dispose of regents then I believe it would take away one of the challenges of the game. Once my regent became infertile I built another keep and recruited more heroes to start a new bloodline. Once she died I married her only child to a young hero and the bloodline continued.
Had there been a way to remove the regent then the whole strategy portion would change. So it would be nice to stage a coup d'etat, there would need to be some penalties for it.