I think adding most memorables soundtrack from Halo 5 campaign to Warzone Firefight would be awesome (The trials (the best), Advent (beautiful), Walk Softly (Mantis), Osiris suite Act 1 (really epic), Osiris suite Act 3, Scarvengers, Light is green, Sentry Battle, Covenant prayers (against Covenant bosses), Cloud Chariot (would work very well), Genesis, Crypt (intense), Kamtchaka, Merdian Crossing, Cavalier (could work very well also), Warrior World.
Maybe it is not easy to do (because rounds are 5 minutes, and musics would have to stop randomly during rounds… An easy solution could be to add soundtrack only during the final round against the bosses). But fighting dozens of Covenants and mythic Warden Eternal with a Hannibal Mantis on The Trials, Advent or Walk Softly would be a great addition on my opinion. At least in the last round when Spartans are making the final push.
Moreover It could be optionnal.