A base defence mode could be fun. Earn credits each wave for kills, extra for mining resources at each level, spend them on turrets and such for the base. Could be a base that drills down and mines as it goes, each time it drops into a deeper cave system, a new wave begins and you have to defend it while it drills, or perhaps have to repair it in various places to move on whilst also defending it.

Of course, what it REALLY needs is a battle royale mode, obvs /s