You mean the mercenary band getting xp when winning battles doing quests and so on and therefore getting little utility bonuses? This sounds interesting.
This could then be supplemented by the choosen followers.
"I am a Paladin!"
>OMG, Malthus, there are no damn paladins in Battle Brothers...<
"OK, OK! Then I´m a wrecked down minstrel drunkard pretending to be a paladin, singing so wrong in the midst of battle that even the undead run in fear... Better?!"