Firefight was a fun PvE mode to play in ODST and Reach (and 5 I think?) but it could be MASSIVE if the concept was expanded upon.
Imagine if every map was built like COD Zombies maps with unlockable areas and an Easter Egg quest. Or like a Destiny 2 Dungeon with puzzles and big boss fights with mechanics which require team coordination to complete.
Having a PvE mode with the same level of depth and variety those experiences offer would transcend Halo’s past PvE offerings and people would look forward to every map release to be the first to find every secret and beat it on the hardest difficulty.