What I'd really love in a TD game is a decent variety of baddies even within a single wave. I find the way 99% of TDs send out a single type of baddy in single file to be so weak and annoying...how about sending out a random mix of baddies of ramping difficulty (the variety getting wider and wider in the later waves).
Tower Madness has just been released, at least in the European Store Introductory Price (for a short time) is 1,59€!