Wolfstein and DOOM were the first FPS's and the basic formula for FPS was set. Move left or right, back or forward, jump, shoot. Then Marathon by Bungie created a new standard with being able to LOOK left, right, UP and DOWN lending more of a 3D feel and new possibilities to strategies and gameplay.
Beyond the basic mechanics of movement and shooting, a lot of what has been added over the years with COD and Halo has more to do with the genre of either the military shooter or the space shooter—new weapons, new "armour", new abilities based on the added tech each has brought. Along comes Destiny and Titanfall and they add new stories and backstory with their "armour" and weapons, abilities based on the world and backstory created.
So,really ever since Marathon introduced true 3-D feel, movement and environments, there has been little innovation in the 'Mechanics" of the FPS other than increasing speed, increasing jumps, and increasing climbing abilities because the underlying movement is limited to the physics of a 3D world - up, down, left and right. Also, FPS is limited to using a projectile or melee (with a weapon or not) to kill.
Again, the basic mechanics are all the same but there have been vast improvements on implementing the mechanics-less glitches, smoother visual movements, better response, etc.
So how can FPS games be better than the competition? How can they truly innovate? That's tough because they seem to be all following the same FPS formula and therefore are limited to that and becoming more similar rather than different.
One thing HALO had was a whole unique backstory, legend, universe. To be successful it must embrace that and work with it and expand it. That's what will set HALO apart from Destiny, COD, etc.
Make HALO better? Hmmm...we all have our opinions on new abilities, armour, weapons, maps, etc. One thing I think the community seems to agree with is that "more" is not the only answer- i.e just more maps, more armour, more colours, more weapons, etc. We desire innovation and creativity in implementing the HALO universe and experience.
Let's look at a potential innovation in movement- remember teleporting? That was great for adding something outside of the normal 3-D movement that you could implement into a strategy for dominating a map.
What about "phasing"- the idea that you could temporarily reduce your "matter density" to be able to pass through solid objects? The penalty on bad timing of this ability would be instant death is stuck in a wall or other solid object. But, it would be cool to run at an enemy, phase through them to come out behind them and assassinate them. Or, sacrifice yourself be phasing and then res-solidifying inside them killing them and you. Or, in a fire fight, phasing for a second to allow bullets to pass through you giving you enough time to either shoot them first or move to a strategic location on a map. Now, this ability would have to be severely limited in time and scope to prevent overuse, but here is an innovative new movement to enhance.
I think—again my opinion, hopefully in part or wholly similar to others—that HALO needs to be HALO and embrace it's universe. Build on the stories and characters and create new solo experiences. For multiplayer, maybe more or larger or bigger is not the best idea for "better". Better could be better implementation of ideas and playability. Make better maps that encourage and facilitate the style of game-play intended- i.e. an arena map that works well for slayer, or CTF or KOTH or others, not just take a Slayer map and put CTF in it. Think about BTB or WZ and their goals and make maps more appropriate for the gameplay. Make matchmaking, servers, fair gameplay BETTER, not bigger, larger, etc.- just make the IMPLEMENTATION and PLAYABILITY BETTER.
343i- to make HALO better, just do things BETTER, sure add variety and new items, but stay true to the HALO world and focus on doing BETTER at implementation and playability than COD or Destiny, DOOM etc. and you will have the thanks and loyalty of the HALO community. Again, my opinion.