Maybe I'm too demanding of developers, but this is what I would like to see.
Refine the powerups of halo reach and add a few others, but remove armor lock. Make missions in a way that your choice of powerup dictates how you complete it. Mission has you going up against an overwelming force single handedly? Pick up the invis powerup and sneak past what you can't handle. Pick up the jetpack and go through the level in an alternate airial platformer setting. Pick up a new juggernaut perk and try to go through the enemy mass like rambo. It would encourage replayablity as the stage will play completely different depending on the perk you used. Obviously not a perfect example, but you get the gist of what I want. Genuinely different ways to complete a mission.
Branching campain. You decided to finish the previous mission by flying over everyone and now the enemies you didn't defeat ended up doing something they wouldn't have if you killed them. Now you have to defend the town full of innocient aliens they had taken over. You decided to go rambo and took your sweet ass time killing every enemy you saw, now the enemy had held you off long enough to accomplish their goal, and you are taken to an entirely different mission. You do a mission with a time limit and if you run out of time, it isn't game over. The bomb that blows up just makes you fall somewhere and do a mission you wouldn't have had you been able to defuse it. Basically make the campaign actually be a different experience every time you play it. A 7 hour campaign wouldn't be bad if you can play it again and again and experience different missions and an altered storyline. It is bad if once you beat it once alone and once with friends, there is really no reason to play it ever again.
Pretty simple. Have a playlist including the powerups and have a classic playlist for when people want to play halo like it was in the original 3. Expand Invasion to allow for more players and more interesting objectives than just capture point a, then point b, then return the flag to your side. Have a competitive firefight mode. Two teams compete to try to get the most enemy ai kills through any means nessessary. Make a forge matchmaking option. 4v4. 4 rounds. Every round 1 person is the forge robot and has a limited amount of money. Goal is to summon the nessessary platforms, vehicles, weapons, to allow your team to complete the objective, whether that is as simple as team deathmatch, to something like getting the flag thats high up in the sky with no aerial vehicles allowed.
It will be a bad idea to complete change halo, but they have to come up with at least some radical ideas to keep the game fresh. Previously when they released only a game every 3 years, they didn't need to do much. When a halo would finally release people would gladly take anything. Nowadays there are too many halos coming out and the game isn't even the king of fps anymore. They need to make each experience different and memorable if they hope to sell a new game every two year, and a spinoff in between each gap.
Also objective games should not count towards your KD. Far too many people are more interested in their KDs than winning the game. They play objective games like a farm the most kills before the enemy team wins game, because they feel good when they can look at their profile and see a 3.0 k/d. Removing kill/death tracking in objective games will greatly improve the experience as people will be more interested in winning the game than getting the most kills.