If someone can figure out how to do this it might be fun. A map that starts out with: 1. A central circular area with several sets of concentric, circular walkways expanding out from it OR 2. A central platform with rings of platforms expanding out from it 3. A central platform with incrementally larger platforms below it As time progresses the outer rings/platforms or lower platforms drop away, gradually reducing the size of the map and increasing the pace of the action as people are forced closer and closer together. This might work best with a game type with a limited number of lives since the map would be come smaller as the total number of players became smaller.
Another possibility: Jet packs or man cannons only spawn after some period of time, making new areas accessible. Suppose there are 3 areas, the outermost, middle, and innermost. The outermost is the only accessible area at first, then when man cannons spawn the middle becomes accessible, then eventually more spawn and the middle becomes accessible. Design the middle and inner areas so they offer greater advantages to draw everybody towards the middle and keep the action going.
Another possibility: A map of concentric circles with one way shield doors that allow people to move towards the center of the map but not back. The closer to the center, the more powerful the weapons, the more frenetic the action.
Donkey Kong? Would it be possible to make a map with several ramps leading up to a top one where barrels spawn and roll down that attackers would need to jump over to get to the top?
The shrinking map idea might also work well with long map - either have walls spawn gradually reducing the length of the playable area, or have areas drop away?
Another possibility: KOTH map with the hill in the middle of a bunch of one way shields. If the shields are facing IN once you're in, you're in and it's a mosh pit (maybe some cover to keep it from getting too frenetic) If the shields are facing OUT the only way in would be to go OVER, using man-cannons towards the outside of the map. This could be interesting with hammers only and very low damage but high shields or invulnerability.
Ball pit: Arena with kill balls or golf balls and hammers only. OR A map where neither team can reach the other and has to kill them by knocking golf balls over a wall and into the other team using a hammer. I accidentally killed myself in Forge when a grenade I threw blasted a golf ball into me.
Another possibility: KOTH with hammers with hills on raised platforms surrounded by outward facing man cannons. If you get knocked off the platforms or get too close to the edge the man cannons can send you flying away from the hill.
Jousting: 1. Long narrow corridor with one-way shields only allowing movement towards the middle until opponents are within about 10 yards of each other 2. Mongooses (mongeese?) at both sides. 3. Rockets!