My wish list.
1. Keep the game more balanced. As in have maps with power weapon spawns rather than ordnance drops. I don't have a much of a problem with loadouts as you guys have done a good job with balancing the weapons. (excluding the dmr...pretty OP at times)
1.5.Armour Abilities. I think a better way of doing AAs would be to do it more like halo 3 with equipment. Still keeping AAs but having them as a pickup rather than a loadout, this varies the gameplay much more as you are not just using the same AA all the time. and again (at least in my eyes) balances gameplay.
2. Maps... I'm not going to claim that I know much about making competitive maps, but I have noticed that the maps in halo 4 (although they do look stunning aesthetically) do not play as well as previous games, with some exceptions like Haven Solace and Adrift. (Adrift is a very fun map in my opinion...one of my favorites of all time)
3. Skill-based ranking system. Its not very important to me personally but I know that many people want some kind of skill-based ranking system (I don't really need to go in to this one because its pretty self explanatory.)
4. Campaign. It was amazing and I wouldn't have you guys change a thing! The Campaign was One of the best I have ever played (Tied with Portal 2). This is extremely high praise from me as I am a very grumpy-picky guy when it comes to video games!
5. I almost forgot! a FarCry/Cyrsis style forge system would blow my mind!
