I always wanted a Planetside 1 stylization of Colonial Marines. Custom character, expanded weapons, RPG like skill system, various planets and stations to deploy to. Aliens rely on stealth and capture (But can still respawn. Tickets raise for captured marines and thus = more Bots). Marines get various objectives and will require teamwork since going solo around xenos = instadeath unless you're Ellen Ripley (Or Amanda). Vehicles for the marines of course, but are capable of being climbed on, and taking acid damage (And you wouldn't wanna use the weapons around objective as I'd like to see things you need to protect/fix and allies take FF dmg which can lead to failure and allow the aliens to get a boost in the zone. And killing too many allies = 1 day ban from the game
).
So that's my ideal idea for a Online Colonial Marine shooter. Hell, even let players play as Aliens and start from facehugger and rank your way up. Three Highest ranked people get's to be a Praetorians.
My ideal single player would be the canned Aliens: Crucible game that Obsidian was working on but got slapped aside for........ A:CM.....