Just an idea:
Remember good ol' Q1 Team Fort? I think the classic map (2fort5 was the name) would make a fun team-vs-team DnD grand melee, where everyone makes a 10th level character and can hop in and out at will. This would of course require a few things:
1) A pre-made physical map, preferrably in 3-D;
2) A large number of players coming and going throughout the day(s), i.e. the game would have to be at a convention;
3) Multiple judges/GMs to run the darned thing.
Some problems I see that would need to be addressed:
A) What happens when characters die? Do they respawn? Are they DEAD dead? Can the player make another character and rejoin later?
I would suggest that there be 11 pre-gen characters (one for each class) that the players could draw from. This would allow for balanced respawn, as, for one, some of the more game-breaking stuff can be excised by the GMs beforehand. This does have the distinct disadvantage of making the characters a little boring, as they are all carbon copies of each other, and it squashes the variation possible within each class.
Also, I would suggest not allowing resurrection, as the only raise spell castable at 10th level would leave the revived character at only one hitpoint. Though I would allow healing and the "Death's Door" rule, as this adds a nice level of complexity to the game (ex: I just dropped the fighter, now let me camp him and see if the cleric takes the bait! Bwahaha!). Theoretically, the characters could have items that resurrect, but the sheer cost of those items would put them out of reach of the 49k gold 10th level characters start with.
B) How many players on the board at once? How are players removed and new players added?
Here, I would go with eight players, as this forces people who want to play as teams to decide which character types they must abandon. It is also a nod to the original player cap of the map (if I remember correctly, or was it 16?). Plus, sixteen players would get a little crowded, especially with AoE effects.
I would also use a king-of-the-mountain system, where if you die, you are out, and the next person waiting to get in "spawns". I would allow players to re-enter the line and wait their turn to get back in on the match. Because of the re-entry, I would suggest either pre-gen characters or rerolling of new characters so as to prevent cheesy characters made by cheesy players from ruining the game for everyone else.
Going to add more to this thread but logging for the night.