What I would like...

[The Noid]

I'm pretty sure you've all played VectorTD, and the music in VectorTD consists of two tracks. One simple beat that's always running, and a second track that starts once a wave is started.

I would love to take that idea a bit further, with a set of 4 or five layers.
One simple base-beat and 3 progressively more "intense" layers. The music could start out with the simple beat, and add (or switch to) higher layers of music depending on how far the creeps progress through the map and depending on how many lives you have left.

If you have 20 lives left, and there's one boss creep on the map, it would tick to layer one, as that boss would have to travel the map 20 times before you'd die. But if you have 20 lives left, and a swarm of 20 creeps is about to overwhelm your defences...

The problem of course is finding someone to make the music...