Personally, I like the idea of a muliple character campaign. That was a really cool part of Halo 2.
What I wouldn't like to see is the hand to hand combat thing. Nothing against that type of gameplay, but I just don't think it would work with Halo. To me, quick-time events would be a little lame for Halo.
What I think would be awesome though, is number 4 from your list. Being able to branch out as either an elite or spartan in multiplayer would be sweet, and it would add SO much more stuff to do than there already is.
I wouldn't like a "weak point" in my armor, only because it would lead to some really lame kills, and everyone would just aim for it alone. Assassinations, to me, do a good enough job of balancing out that sort of thing.
It would be cool though, to see cracks in your helmet as you played.
As for my input,
I would like to see maybe some destructible environments? Halo is supposed to show off the capabilities of this next Xbox right? I say they should go all the way. And not that lame square-hole-in-the-wall that Battlefield did. I mean dynamic, unique ways to blow up the world around you, so that it's different every time. Multiplayer would get mixed up quite a bit (in a good way) if they could pull that off.
*Halo 4 campaign spoilers*
I'd love to see them go into the history of this Didact guy. I think it would be super dumb if he really just died in Halo 4 and that was it. Even in the Halo 3 terminals, they mention him, and now he's finally here for what? One game? He should come back.
I'd also like to see what happens to Master Cheif now that Cortana's gone. I'm not sure if she SHOULD come back or not (it wouldn't be the same!), but I know I definitely want her to! Haha, I was a huge fan of giving personality to Cheif in Halo 4, and I think they should keep going with that.