Nice ideas.

Heres a example to add this idea, Archimedes adds seige bonuse's,I think its
1+ when attacking a castle and 100 extra building points YET the generals that most likely get this are the ones governing city' with ludas magna, and by then they are to old and to far away from the combat to do anything with the guy.

But on top of this idea I think you should have a pool of potential retinues, say you got a cilviled slave, It should cost you XX amount of denarii to get him/her to Hergoth the Hungry (Or whoever). Then other retinues you cant recruit, like priests...But you should be able to dismiss them.