MW2 Mercenaries / MW4 Mercenaries were basically that. Actually, the first Mechwarrior game was that as well, even moreso than the previous two I mentioned. You could travel anywhere in the Inner Sphere, get jobs, hire pilots, and buy/sell mechs. Apparently there was a Star Control 2-like time limited plot, but I never bothered with it as a kid. I just ran around trying to make enough C-bills to field two lances of Battlemasters. That was the gateway drug to the tabletop game for me (which my brother had already been playing for years).
I recommend MW4 Mercs though, if you want to build up a squad of ultimate badasses and go on cash runs, and also play something that doesn’t look like complete ass.