The map is obviously quite elongated, so having two bases in the middle of the map (wall & pelican), there would be no other good place for a third base, as it would be unfair to the team who spawns on the other side of the map, unless spawning is dynamic, which it probably shouldn't be.
If it were the two bases, then a central base could be built in the center of the map (there's heaps of room near the wall, not so much near the pelican).
That was pretty much all the thought I put behind it.
Trust me in saying that the maps that got in are the best of them. You don't want any more. Invasion is an immensely difficult mode to forge for. There are so many things that need to be not just considered, but analyzed to the bone, just to get a basic layout to then forge, and with the issues Reach has with performance (frame rate drops, budgeting, objects in FoV which can lead to the lighting engine skipping, etc) it takes months to design, build, playtest and iterate upon an Invasion map. I have the utmost respect for the forgers who spend the time to work on them.