This would be a doozy, here we go.
1) The most important thing, a map equivalent to that of Fuel. It would be just a big island, where if you go too far from land, you get your ass eaten.
2) The story; You are a mercenary that is sent off world to rescue a group of scientists that aren't responding to your calls that are on an alien planet.
3) Other things to do: Trophy hunting-I'll get to that-, looking for their research, scanning the fauna and flora-again, soon-, and just finding random shit.
4) Those that hamper your progress: Alien creatures that I would personally have a hand in designing.
5) Weapons: A build-a-gun feature like in Ghost Recon: Future Solider, except you can mix and match parts. Any parts. You could make a shotgun-minigun, a rocket launching sniper rifle, etc.
6) Trophy hunting: you can buy gun parts and ammo, which would be rare in the over world, buy hunting and killing bigger and better animals.
7) Scanning: You can scan in the animals poke'mon style to figure out their weak points, strengths, hunting style, favorite colors, and their favorite Muse song (not really though).
8) Possible multiplayer: Horde-like mode, competitive trophy hunting-limited time, only one kill per round, most points cumulative wins, and bigger things are harder to kill-, team trophy hunting-like competitive but with a buddy or two-, Rescue-go into the wild with a team, find and secure a researcher, bring them back to base, while creatures are trying to see how you taste-, and Hunt-first to find, scan, and kill a certain creature wins-,.