I have to agree with everyone saying that this shouldn't be compared to Call of Duty or other FPS series games.
Call of Duty is done. Infinity Ward make those games and make them well, and there's no way that AvP will compete with them if it's the same game with Aliens and Predators. What an AvP requires is its own atmosphere and its own mechanics, and that's what we've got. I guarantee that if we got a CoD-style game, people would go back to MW within weeks... because Infinity Ward do that kind of game better.
Rebellion have crafted a game with its own atmosphere and mechanics, and in the process it's become something Infinity Ward can't touch in terms of character. I'm incredibly happy for this game to be disassociated with Infinity Ward, not because I don't like CoD games (because, f**k me, I love them to bits), but because it's completely removed from what AvP should be. Different settings, different combatants, different atmosphere, different games.
AvP as we've played it certainly isn't perfect, but my idea of improvement isn't killing its character using examples from CoD. Try to think about whether a mechanic can be objectively improved or whether you're not comfortable with it because it's new to you, and because you want something familiar (such as something from CoD).
Insofar as my opinion counts for anything, no mechanic really needs to be altered completely. It'd be good for fans of the previous games if the Alien options for wall-walking were either 'modern' or 'classic', where 'classic' uses the 'walk-on-everything-as-long-as-you-hold-down-the-button' mechanic. The Predator secondary weapon mechanic was a bit strange getting used to, but ultimately I think it's for the best. Marine gameplay is really solid, woiuldn't change a thing.
The only change I'd make is balance. If Predators do more damage than Aliens, fine, but I think it's a bit much that they knock Aliens on their asses so easily, yet Aliens have such a hard time doing that to Preds. So I'd equalise them in that regard.