DIRECTOR MODE - this isn't really a good idea, but it would be a mode where someone could set traps (like rocket launching robots, spikes etc) on some areas and try to stop someone from finishing a level... this would require some more thinking tho.
Player 2 blocks the path with 8 saws.
Player 1 rage quits.
Player 2 wins.
The idea is still good:
PUZZLE MODE - think about Trackmania with Meat Boy! You get an unfinished level and you have to place a limited amount of blocks to complete it. The person with the less needed blocks wins.