To be honest, I've always thought TDs as more one side of an RTS, you know - Test my Turtle Skillz, but in my own time..
I guess that's why a new TD on the flash sites normally gets a play, at least a try.
I was kind of thinking more Diablo - or Blood & Magic if you've heard of..
RPGs are not only based on levelling up. I for one am a massive fan of exploring and looting.
Maybe one, or select few characters to point & click. Perhaps a mission or loot to discover beyond the cash-for-kill..
Powerups or synergies based on character/tower stats?
Buildable/non-buildable areas?
Just throwing a few things out there..