I would like to see two additions to the game play modes
1) Add a vote for element mode. In the round before a new random element is unleashed, have the remaining players vote. Votes not made before time is up are ignored. Not sure how you would handle sell back rate on this, if it would be %100 or %75, or something in between.
2) A mode where when creatures reach the end of the track, they do not respawn in your area, but in the next, so that the whole map becomes a big circle. This would make it work similar to the DemonCircle TD, if you have played that. Game would end when a certain number of creeps are still alive in game for all players.