Actually, rebalancing skills, and risk vs rewards only requires a patch.
More like an entire overhaul, it would change the game deeply. To be honest whats needed is an rpg system in which you as the leader of the gang increase your scouting and negotiating skills, on top of some strategical and tactical ones. This would allow the gameplay to evolve, youd be able to get better guys, secure better contracts for increased reward, have more options at the start of combat as the game progresses, have better options to deal with the merc company management and even increase your skill to deal with individual brothers.
The possibilities that would have been opened by making this an actual rpg are insane, boogles the mind they didnt go with it while mount and blade did like a decade ago.
Adding new enemies and terrain types could be done in an expansion.
Unless you need quick cash i wouldnt bother with an expansion, a sequel is a more interesting prospect, because you get to iterate on the mechanics and gameplay in a more radical way. Im betting the team learned a LOT making this game, instead of expanding on it slightly just take everything you learned and make something better.