2 playlists that came out of my head that might be good.

Games Other Halo
1

1.Great Invasion
8v8 Invasion with 4 sets increased time
(Optional:Make players spawn with 1 grenade to stop easy grenade spam)

2.Team Triples
3v3 slayer and objective games.

What do you think?

2

I thought of a new one!
3.Squad Objective
5v5 Objective games

3

Not worth splitting the populations.