Maps that are mostly Forerunner built, or an arena all together.
Maybe there could be maps that have you take place in a massive building, and have lifts that can transport all your inits into more floors, kind of like in the Halo Combat Evolved level: Library, where you follow Guilty Spark through the Flood outbreak as you go up different levels to reach your objective.
Have a game mode called contamination, where it’s a regular team battle, but in the middle of the map is a massive Forerunner base that practically cannot be destroyed. Around the map there is enemy Flood units and structures, and killing Flood units will cause them to explode and contaminate your units with a Flood virus which does nothing to you, but causes the Forerunners to focus the units that are affected. So in a sense, you capture these Flood units and round them into your enemy base and force them to explode which makes the Forerunners focus the base affected.
Any other ideas? Post them here