Well… For all I know, as soon as that happens, someone will make a WW2 gun mod XD …
Actually I think it’s far more likely they’ll introduce muskets/flintlocks (terrible accuracy, lots of power, reload on fire similar to the crossbow), or maybe a basic repeater (no reload necessary, 3-5 rounds, terrible accuracy, lots of power), as an upgrade to the crossbows. WW2+ style weapons just would seem so out of place in a medieval style game. The advance of early guns would make a much better fit, although still a bit of a stretch.
As far as your regular suggestion goes, it could work pretty easily if there were more nests of monsters. For example, if you wanted to play an orc, you’d need more orc nests in the game that you could visit to receive contracts from/recruit battle brothers. Towns would then act as extreme monster nests because of the spawning of militia near them. You may even want to stay off the road unless you’re required to, to raid a caravan. So basically it’s the same game, just a different perspective. I wouldn’t mind them doing this, but it seems like an awful lot of work to develop contracts for each monster faction, when there are few enough to be had for the human faction. I think this type of thing will be something that will probably happen either as a mod, or as expansion content (should there be any).