I think stunts would be awesome. Say, you got a guy around a corner. He's got some huge weapon and it's the pain. WEll, what's the least thing he'd expect? You diving in front of him stabbing him with bullets. That'd be awesome.
Umm, Zero-g jet pack or ship battles would be awesome too. Get a saraph and unload hell on a USMC Frigate. That'd be cool. I wish you could chunk things, like hurl dead bodies at people, or something like that. Or think running at someone with a sword. What's the last thing HE'D expect? The sword getting thrown at him.
Of course new weapons, like a minigun. Makes you walk slow, you can't jump, but it's really powerful and stuff.
A knife. Yeah, a knife, my favorite and best weapon (in RL too

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Bots. Definately, so annoying when you join a game, sit there for ten minutes, then the host leaves since no one else comes. They also need an optimatch, where you get to choose your game. Not always get stuck on King of the Hill on Zanzibar every time. Well, sometimes its CTF :/
You should be able to snap peoples neck. Or something like that, like, head shot melee, bam, knock his head off

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Narq-Gun. ......drugs

, nah, j/k. It'd be cool though, to like hit some one with it, it slows em down and blurs their vision, you just run up behind them and stabthem..... with a knife. Or beat them with oranges. Both work.
Umm, being able to light things on fire. Big time. As in like, moltov cocktail at a grunt.

Or a flame thrower.
Sentry guns would be kinda cool. Like, you get one that you can set, it has limited ammo. It'd help alot on legendary. Just chunk it out there and let it clean up.
And submarines. No game is complete without em

If I think of anything else, I'll tell ya.