While we can hire a fairly decent staff already, why not add to it. Enable the ability to train the constable, so he can train other troops faster. Perhaps a doctor, to increase the heal rate of you/troops whilst inside the city he is in.
Also, I know many of you would have seen the siege weapons (the trébuchet, for example) that are deployed during sieges in the game, however they are never used (they sit idle). Why not add an option to recruit siege weapons (mercs, perhaps, or maybe you are required to build a siege facility in a city and recruit from there). You can use the model that is already in the game, add soldiers standing about it and effectively make it fire the grenades from M&B WFaS. Just an idea, and I know, it is a decent task.