You know if they made this game sort of an MMO/FPS game it could be epic. You have the central city where its server based, and all avatars can wander the city with shops, AH's and such for different weapons and armor. Different Clan Halls, Guild Halls, and job recruitment stations. With the storyline you can play single, or Id say up to 4 player co-op. After you play the campaign is when the city becomes accessible and you can find groups for other random missions. Full character customizations with rare armors and weapons. The different jobs you can take would be like Halo 4's Spartan Ops. Throw in some Firefight from Halo 3 as defend type missions with rewards similar to Mass Effect 3 multiplayer. Future expansions are new area's taken back as you fight back against the alien threat. Like a jungle area, desert area and so forth.
As you grow in power so do the enemies get more powerful. Mowing down waves and waves of easier infantry who you struggled against at first and then fighting what used to be bosses earlier on but are now mid level enemies as you fight stronger bad guys. Viewing you more as a threat as you claim back more of your world.
Maybe even throw in some form of organized Clan play where clans can que up for competitions and such. 16 vs 16 payer clan fights in arena's.