This is a great FPS in general, let alone a great Halo fan game. It's amazing what this engine can do when pushed to this degree and the game provides lots of options for replayability.
Two things I would suggest are bot difficulty settings in Offline Multiplayer, like seen in the Firefight mode, and pathfinding updates to the AI. As it is, the bots like to peer through even the smallest gap between structures when aiming, making them unnaturally accurate, and can often have trouble moving through the maps. Allies can occassionally become useless as they simply refuse to pathfind from time to time, while enemies often stay clustered in a single area.
Likewise, the ability to play Firefight vs Covenent and Flood on more or all of the maps would further serve to broaden the experience. Each enemy type is currently limited to a single map and this is quite limiting in terms of gameplay.