I think the idea of the map size is to allow for players to go off on their own without major combat all over the place. Because think about it a bit, the reason for the Flag in CTF being at the very back of the base is so the team would need to be together to go and push back the defenders and grab it.
Same goes for CP maps, you normally narrow it to 3 routes that all connect back into one larger area, which is normally where the team defends the most.
KOTH maps are built so you have to defend at mid most of the time.
But with this game mode, you need those larger areas to allow for more free-flowing combat so the robots can be destroyed, and the game actually plays out instead of just making the entire area get locked down by a single class, which granted the engi can do already; but at least allows for other classes such as scout, soldier, spy, and demo to be more effective in the point grabs.
If you were to work more as a team, it also still functions to where the game would end much quicker. But if you're in the mood for a long game time, it's a slow process of sending in the troops to take on the problems for you. These are the classes I've come up with for this type of gamemode broken down into a 16 player team setup.
Attackers:
Scouts: At least 2. These guys will be the ones you send in for the purpose of grabbing the cores early game, and collecting points for the team. They will tend to change position to grabbing the battery to get points back near the end or middle of the game.
Soldiers: 2-3. At least 1 shotgun, 1 Buff Banner, and 1 Backup/Concherer. They'll provide sentry destruction backup for the scouts allowing the scouts to grab the cores or battery depending on the situation.
Spies: Really only need 1. Anything really goes with this class, only goals I'd set with him is taking sentries down in the absence of soldiers and getting the battery out into open area for the scouts to grab the battery easier.
Demomen: 1 or 2, depending on issues. Quick, hard to reach sentry placements destroyers. Only when the soldier can't hit the sentry gun, or when the there's no soldiers to attack a sentry gun with.
Medic: Only 1. Either the Quick-Fix or Vaccinator medigun. (Maybe Kritzkreig) These are practically built for taking on small waves of enemies and can lay quick work to a sentry gun's bullets if given enough room.
Defenders:
Heavy: Only 1. Used to lock down closer combat upper areas, to prevent players sneaking around above the team's head.
Medic: 2 needed. Heal teammates, the usual. Preferably default medigun, Vacc, or Quick-Fix. Whatever one works the best in your favor.
Engineer: Preferably 2, no more than 3. Defend the areas, keep the enemies out of your base and keep them from stealing your batteries/cores. Best setup possibly would be to have at least 1 sentry gun in the back, and 1 in the middle room with the water on the balcony with a wrangler.
Pyro: 2 or 3. Keep the close combat in the transition rooms for you to get the best effect (obviously...) on enemy players. Try to keep at least one in the cave area of the base .
Sniper: Only 1. Good sightlines in B's area for you to kill oncoming attackers from a distance, use to your advantage.
I know people are gonna disagree with this, but me and my friend's tried this earlier, and it worked fantastically over the course of 7 games. It was almost always a nice paced game, and wasn't over too quickly, but wasn't too slow either. I know in pubs this probably won't work. But in terms of actual cooperation, it works great.
