I'm liking these Ideas. Here's one of my own:
Place able AI spawn spots, and different spawning bots.
Example: Say you make a symmetric capture the flag map for two teams. In the middle are A.I. in the form of Human, Covenant, Flood, or Promethean whose objective is to basically impede your progress at obtaining YOUR objective. This could make for some intense three way battles. Expanding on this idea, is forge able firefight maps.
Another thing that would be cool to see, are some wicked multiplayer spacebattles. Like, one map could be right in the middle of a fight between UNSC and Covenant ships. On foot battles can take place in a debris field and some of the vehicle selection could be Broadswords, Pelicans, Seraphs, and Phantoms. Adding in the ideas from earlier, and we've got a three way battle on the backdrop of space, explosions happening everywhere, and the player feeling like they're actually in a galactic war.