Soliciting System Advice

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I'm stuck trying to find a system to run a campaign idea in. I'm looking to you fa/tg/uys for advice, both helpful and retarded.

The concept I'm working with is that there will be 5 pre-built characters that my players will control as a group. They'll vote on each character's actions as a committee. If they can't agree, the character hesitates and loses his actions. Each character will have personality traits and flaws that may cause them to act against the group's wishes anyway. They're going to be explorers hexcrawling an alien planet, looking for resources and alien artifacts and such.

I'm looking for a system that is complex enough to allow the 5 explorers to specialize (as Tank, Striker, Crowd-Control, Scout and Support), broad enough so that they can upgrade to cover their weaknesses, and enough tactical depth that the players feel like they're not just rolling d20s all night. Ideally, the system would also allow for upgrades from alien tech.

At first we were going to use Mutants and Masterminds, but it became apparent that tactical combat doesn't work in that system. Then we looked at GURPS, but it's just too dense for the kind of pick-up game it's supposed to be.

I know it's not an easy task, so, what do you suggest?