I haven't played any MMOs at all, but I do see potential in a Halo MMO. Maybe not a MMORPG, but maybe just a big freaking battle akin to Planetside 2.
Now here's my plan. There will be multiple gametypes. Here's one.
1. Strain - maybe 40 - 200 players?
Beginning 2 playable Factions - UNSC, Covenant.
Enemy AI - Forerunner Constructs (and maybe Forerunners themselves), Flood, and indigenous wildlife of The Ark or the Rings.
The setting is on The Ark (or any of the respective Halo Rings). The goal of the Covenant is to successfully activate The Ark/Ring, and fire the Ring(s), and the goal of the UNSC is to stop them and destroy The Ark/Ring, and flee the impending explosion via escape Frigates.
The Flood crash on the Ark, or Ring (via High Charity if it's The Ark, and released from containment if it's a Ring) about 25% into the match. If a player of either faction gets killed by Flood they turn infected, and attack the 2 Factions.
The 1st player turned into Flood gradually begins turning into a Gravemind (not the imoble Proto Gravemind, but a combat ready mobile Gravemind). If they leave the game the 2nd player turned takes their place and whatnot. The Gravemind is a unique form that allows the player controlling it to cross the battlefield in a Halo Wars esque control scheme to control AI Flood, and direct Flood players. The Gravemind, after reaching 50% of the Player population turned Flood, and then leave the Halo Wars thing and go into battle as a fully formed Gravemind. If the player decides to do this, the AI Flood go feral and lose composure and combat tactics. They can still attack you.
The Gravemind is a slow, but powerful combatant. It can destroy enemies instantly in a body slam. The Gravemind can also tunnel underground to move faster. Flood AI In the area will surround the Gravemind and act as protection for the player, and the player can command the local Flood to attack specific targets.
The Gravemind is not invincible. If it sustains enough damage, it will die. The role of Gravemind will then be given to another player: what specifics can be modified.
What loadouts a player can choose. You unlock a different loadout after a certain amount of kills:
The Flood players can choose from different Flood forms, like the Pure form (Tank, Ranged, and Chicken), Combat form, and faster Carrier form, and maybe a swarm of Infection forms. Possibly even infected wildlife if you manage to kill them as an infection form. The Gravemind is exclusive to either the first infected player, or the highest ranked player in the match that is turned initially.
The Covenant players can choose from the different Aliens of the Covenant. Bugger Captain accompanied by a miniature swarm, Hunter pair, Grunts, Jackals, Skirmishers, Elites (different ranks), Engineers, Brutes (different ranks) and such.
WHEW that's all I got right now.
I know I didn't include some information, but this just here for perspective on what an actual Halo MMO may look like. Sorry for the crappy grammar and spelling. I'm on mobile.