Disclaimer: My Opinion
In my personal opinion, I don’t feel like Halo 5 really brings anything new to the table. Warzone sounds interesting, but when you actually play it, it doesn’t really feel new. Fun? Definitely! But again, it doesn’t feel like anything new. If you disagree, that’s cool, but I also think it would be cool to come up with some new ideas to ask 343i for. As far as gametypes, and overall new ideas to Halo. Going to list some (Big or small) and you guys feel free to add some ideas!
-Space battle Multiplayer: Two teams with two opposing space stations, you have to fly a ship to the enemy’s station (with some cool space combat on the way), set a bomb, and blow it up. First team to blow up enemy’s station wins. This would feature infantry combat (On the stations) and space combat (With ships outside the stations).
-Water-based combat: Create water vehicles (vehicles that can dive underwater) and just include them in general combat in BTB and other game modes. It would change things up a little bit and add a new dynamic.
-Stealthy campaign missions. Like, you have to sneak around enemies during levels. That would certainly add a new dynamic to campaign.
-Firefight: Not new, but… Cmon… 
After every 3 rounds of a firefight, you face a boss of some sort, or something.
-Kind of an addition to firefight, more like a defense mode: AI’s try to destroy your base. You have to successfully stop them, or you lose.
-The ability to add logos/patterns to your Spartan armor; that way you can REALLY personalize it! You could place logos on arms, legs, chest, back, helmet, etc. As you progress through the game, you unlock logos/patterns.
-Add MLG/Hardcore Playlists, then add the ability to search and spectate other’s games live in the game.
-Emotes, maybe? Lol.
-Ability to set up computer-controlled Spartans/Elites in Custom Games; that way you can practice alone, or just set up big matches with a couple of friends. You could set their difficulty, which would obviously affect how effective they are on your team, as well as the opposing team.
-New skulls: A skull that turns every enemy in campaign into a grunt, a skull turns every bullet into a grenade (LOL imagine that), a skull that gives everybody (including enemies) shotguns, etc.
-Dynamic “race-against-the-clock” game mode. Whether it be with your spartan abilities, with vehicles, or both, etc. You could have a competitive leaderboard for this.
Anyways, just throwing random ideas out there. I know these aren’t exactly unheard of, but they would definitely bring a fresh feel to the table, in my opinion. What do you guys think? I’ll add extra ideas as they come below. vvv