the idea of a heavy wave is an interesting one, i think there is a way to make it work
the main problem with a heavy wave is the drop in WPM, so, to solve that, the game could actually force you to fight against 2-3 rare waves in a row after the heavy wave is defeated, compensating for the extra time spent on it, maybe a message of sorts could appear, like "the death of [monster name] made some enemies flee in terror"
the dragon wave is also nice, surely, it's a DA only option, but i don't think it's a problem per se, the most important thing is to give enough variety so that people aren't worn out that easily, if it can be done, than it's ok (of course it also means more work for staff, but if they have the spare time, i don't see why not)
on the waves giving more DM thing... there is a way, though that would need some rework on the DM items cost, it would work using a "super defender medal", the SDM would be the equivalent of 5 DMs (so, there would be a merge shop that would merge 5 DM into 1 SDM), the wave that is meant to give more than the normal amount of DMs would give one SDM instead
the problem with this suggestion is that item prices would need to be changed from DM to SDM, also, since we have no multi-merge system, it would make it really boring to merge 5000 DMs into 1000 SDMs...
so... i thought it was a nice idea but as i was writing i just noticed more and more problems with it... guess i should just scrap it, i'll keep the SDM idea in the post just for the sake of saying that there are some ways to trick the system as pyropuppy said