The following suggestion won’t happen unless Big Team Battle becomes a social mode as it would skew player stats in ranked arena mode.
If you go back and look throughout Halo’s history, there were always ways to cater to the casual community (dabblers too) and others looking to relax with some simply silly game types/modes in multiplayer. It mostly happened in Big Team Battle. Now I understand that many do stuff like this in Custom matches, but that is invite-only. Halo has always had its foundation built on arena principles and thus the potential it has to expand its outreach is much greater than a traditional military shooter.
In Halo 2/3, there was always the occasional silly option thrown into the voting process to choose from. Here is an example from Halo CE on PC: Prisoner | Halo Alpha | Fandom
“Some players make a 16 person CTF servers with only rocket launcher, fuel rod cannon and the flamethrower. This creates a fast paced, chaotic but balanced game where players never survive for more than over a minute.”
In Halo 2 and 3 there were game types like Rockets-only, Snipers-only, Rockets/Lasers only, Heavies, etc. H4 had Gungoose CTF in Big Team Battle, which shows that the concept has been on their mind in recent times. What about matches with Swords/Hammer-only? Covenant-only weapons? etc etc.
Variety is a big key for dabblers and casuals. It allows them to play with a variety of weapons and game modes. Its just silly fun. Sometimes silly fun can go a long way in player retention.
Any other ideas?