One thing I disliked from previous firefights the constant change with skulls. I felt like that was limiting the freedom of the player’s creativity in surviving, then things just become tedious. I especially didn’t like how enemies in Halo 5’s firefight were just bigger bullet sponges. It was just a pattern of dropping heavier loads of vehicles the longer it plays out.
Difficulty changes were fine, since it didn’t limit how you played, just challenged you to play smarter. If there was a way to make it more challenging by gameplay, I prefer just the enemy to be constantly upgrading their arsenal, and amassing the number of enemy units.
Though, nobody really asks for firefight to change, so this is just a personal opinion by me.