Black Ops 2 got away with weird, realtime strategy sections, a branching storyline, and a handful of non-linear levels. So really, Infinity Ward has no excuse not to try something original. They invented Call Of Duty, after all, and now these upstarts are the ones coming up with new ideas? So far, the most revolutionary thing IW has shown is a dog.
Structurally, IW has already confirmed that this is still a linear shooter with a few more open levels. So let's be realistic in our requests. Rather than scripted, Simon-says stealth sections followed by scripted sieges and scripted chase sequences, why not put these supposedly more open stages to good use? In Crysis 2 and 3, Crytek's sandboxy "combat bowls" were a disappointment, but in Call Of Duty they'd be a revelation. The CoD games often hand the player some interesting toys: it'd be fun if we got some say in how we used them.
Our other suggestions are directly inspired by Infinity Ward's own trailer, so they've only got themselves to blame. "We're All We've Got" is the trailer's touchingly earnest tagline, reflecting the idea that in this universe American military might has been reduced so significantly that the team must rely even more on one another. So how about some basic squad mechanics, Infinity Ward? The ability to direct and position team members would be genuinely useful on higher difficulties.
Then, there's this whole "Ghosts" issue. We've already had Black Ops, a series named for covert stealth missions, devolve into all-out war by the end of the second game, but if the trailers claim that "soldiers stand against their enemies, but ghosts haunt them" is going to have any meaning whatsoever, then this game needs improved stealth mechanics. We want to hunt these villains, Infinity Ward, not just crawl after another team member with the word "Follow" hovering above their head.