An art of denfense mod would be fun, huge armys of creeps spawned at you in waves with no enemy demi gods, could be a blast =]
You should have rows of towers and a single demigod to defend them. It starts off with a single Archer and a comment like 'Wow that was a piece of piss huh?'
Then add some more archers. Then Minotaurs. Then some angels. Then mix it up a bit. Add a single wave of preists. Then a small mix and keep increasing while half way through have a single Giant wave. Then add lots more guys.
All the time have your guy able to buy citadel upgrades but nothing else. If you die the next wave gets Catapults
Then throw in Giants and then Demigods and finally have the last wave as 100 Giants and 4 level 20 Rooks with Tower upgrades, hammer slam and structal transfer and the All Fathers Ring... and maybe throw in a Hybrid Beast as well for kicks