In Halo 3, rank actually mattered. It was the only game that I can possibly think of where rank actually mattered. You would get matched up with players close to your skill level and unlike other games, someone who was a high rank didn’t just play a lot but also had skill. I would like to see a Halo 3-esque ranking system in Halo 4 because it worked extremely well and I’m pretty sure many would agree to this.
The idea of ranking up and unlocking items for you Spartan doesn’t seem much like Halo but I’m reserving my full opinion until I’ve seen it first-hand. Getting achievements or doing certain challenges would be a pretty cool way to unlock armor instead of simply purchasing like in Reach just because you played a lot and earned the credits. Also, I’m pretty sure other fans would like armor dlc, not for a price, but applied as game update to add more armor modifications and to unlock the newly added armor through the aforementioned methods.
As for gametypes, I’m pretty sure 343i has some neat ideas in mind, but I would like to see classic gametypes to return like they did in Reach. I’m sure there are some fans that don’t like armor abilities and want a classic experience. Armor abilities aren’t bad, but it adds a different level of play style which I find enjoyable but everyone likes the classic Halo so, why not have it?