The friend system would work best if you have to call them, and they never call you (although they could send you a text msg every once in a while to remind you to call them). Except maybe certain people you befriend will call you if they're in trouble and need help (think of a more dynamic version of when Roman called you from the trunk of that car). If you want to get their "friend benefit" faster, you call them more often.
I really hope that weapon customization is in. It looks likely, since two guys in the trailer have scopes on their AKs, and the other has what appears to be a tactical flashlight.
And I want the jetpack back, even if it's only as an after-story bonus or prize for 100% completion/package collection.
And co-op. I think that's why there seem to be multiple protagonists - when playing co-op, you control one of the guys, and your co-op partner controls another. 4-player co-op would blow my mind.
And finally, I'd like to see more player-initiated side missions (kidnapping/robbery/etc).