We need Firefight 3.0 in Halo 5 with the ability to:
-Have any enemies from past Halo games, heretic covies, brutes, flood, sentinels, prometheans etc... and be able to have multiple factions fighting you and each other at the same time, or make them allies. Or to do something completely random that would have enemies from all factions fighting against you in a random mixture of enemies
-Maps that have day and night and different weather settings
-Throw in a few objectives to complete every few rounds like Mass Effect 3's multiplayer
That is what I would do to improve firefight