I personally like the warzone mode from halo 5, id love more modes like this. i think that the game could satisfy a whole new group of people with some PVE content.
i would like for large maps and multiple battles across a map where you can fight AI over objectives and other things.
I made a post about this a while back. More PvE is always good imo as not everyone wants the sweaty hands associated with traditional multiplayer and prefer the co-op aspects of Halo like campaign and firefight.
In addition, in my post I also said it would be nice if there could be bots in custom games. I think this would be great in adding longevity to the game (original Battlefront games are still playable to this day due to the fact you could basically play the entire game offline with bots). Also I think it would be great for people who want to say play a BTB custom game but may not have a full party.
Relating back to OP’s post, I have seen mods around for Reach on PC that allow AI to be added to forge as well as game modes that primarily have the AI with objectives etc. 343i could draw inspiration from these absolutely great mod creations.