I kind of wish the player character got as big as the blimp either on a regular defeat or drinking the gold cola at the end of the round. I would like to see the player character become completely spherically inflated!( that is just me thinking out loud about the spherical bit) Is there an end to final hard wave or do enemies keep spawning ensuring the player is defeated? I like the game over all though. I have played it through twice now and enjoyed each play through!
The hard mode is not much different from other modes.
The few differences are the number of enemies and the types of them.
But... maybe yes. There is an "endless mode".
Rounds 8 and 10 in hard mode are actually very difficult because of the amount of enemies, and if you don't blow up the blimp fast enough, your PC will start screaming.
If you want something like "keep the enemies coming! until i explode," you could start a wave with a blimp and don't blow it up.
Theoretically, the round would never end.