Hmmm, an enhancement of the Experience system, and perhaps some character building options for your underlings, I like that. It definitely would add to the Dominions fun if you didn't have to choose between being solo, or having a thousand men..... Running a fearsome mercenary company could be a blast.
Interestingly, if you played as a mercenary, you wouldn't have any single god to ally with, and people would pay you huge amounts of gold for your services. Downside - don't allow mercenary characters to be Sacred (unless you wore a Shroud of the Battle Saint!).