I have a map suggestion I made a bitmap map but couldnt upload since it was too large anyway here is the concept i have (probably impossible but maybe some ideas can be stolen from it)
Castle defence Players 10.
Offence 6 players
Offensive players have no towers or inhibs, the large creeps are always coming at them the whole game. once they hit the end of the trail they die to the shop steps. (the creeps)
Their job is to take down the enemy base.
Defence 4 players.
their job is just to survive against uneven odds. they have towers and creeps to help them out. the enemy creeps are the basic creeps unless the lose an inhib (like normal)
there is no inhibs or base for the defence to take out, the way they win would be by time limit (lets say 45 minutes) so they have to win down by 2 players but they have towers and creeps to help them out.
Jungle, since the defence will just be defending chances are the champion build up would probably be less junglers on offense and more on defence, since defence really doesnt ever need to leave the safety of towers.
Again i built a map design but couldnt post it but its very simple defence in center and offence can come down 4 paths one from each corner.
P.S. I know this will never go through but I love the concept of warcraft 3's tower defence.
another 2 ideas,
1. do just that, tower defence and just make stronger and stronger creeps for some Coop.
2 make it a little like demi god and add the option to buy updgrades for or better creeps.
Please dont flame/down vote me too hard its just some concept ideas very rough draft.
TL;DR i know i know sorry