here is a small list of the things I would do
1. remove killstreak rewards rewards. Instead, reward players with XP for being on a killing spree, and granting more XP to the person who kills them
2. all weapon content is unlocked at start. all weapons, attachments, and perks are unlocked at level 1. the rewards you receive from unlocks will be purely cosmetic, such as skins, color changes, or emblems.
2a. that, or open a shop system with no level requirement, meaning you could get whatever gun/perk/attachment you wanted at any time
3. reduce the auto aim. the auto aim in call of duty is really ridiculous atm, I would reduce it so that it will snap to face a player, rather than snap AT the player. it would move the cross-hair into their general direction, but still rely on accuracy to shoot things
4. Start from scratch. each call of duty game since 4 looks fairly identical, I suggest that the game is started from scratch on a new engine with better lighting.
5. adding an XP penalty for sitting in one spot all day, this would discourage camping and would support the run around whiz bang shooty fun that is call of duty
6. add an XP bonus for trying new weapons often, this would help reduce the whole "one gun" situation where one weapon in the game is seen all of the time because it is better than the others.
7. put the AK-74u back in the multiplayer, that gun sounded so cool...
8. increase the XP gain from assists, this would help players in a game where one guy essentially hogs all of the kills, and in return, all the xp