A new map...

how about simply making Halo , Halo 2, and Halo 3 levels 5 times bigger

For example, the level Halo, from....Halo! Bases, spread througout the area, Hooks, the 4 structures with the big blue light, they could be supply elevators

or maybe, The Ark. that level is huge anyway. make it 5 times bigger, add more areas and canyons, youve got a 3v3 map with tons of extra stuff

and, as a cool tidbit, you can find actual locations from the levels, like the Forward unto Dawns Landing zone

finally, Assault on the Control Room. Need i say more.