A suggestion for another thing to spend $ on: Music

I LOVE some of the mood music for the zones, however some of it is... not really to my taste. I suspect that's kinda the case for a majority of the players (can't please all of the people all of the time and all that). If you had a more diverse selection of music, you could sell the various musics and allow people to associate certain songs with certain tilesets. It might be a bit of a hassle to set up (having a special menu to associate each song with a certain tileset), but it's the internet; people pay $ for songs they like. You could have samples that people could load and listen to on the website, or maybe a small applet in-game. Just a suggestion for something I'd like to give you guys some $ for.

You could bundle "albums" together, too... like "fast paced" or "creepy ambient" or "lots of drums" or whatever, with multiple variations so people could have slightly different music for a huge variety of tilesets, but still stick with a theme for a large variety of levels/maps.

Also, maybe as part of the music purchase, a digital download in MP3 of the music purchased, just in case the purchaser wants to listen to it as background music for other things.

Did you say microtransaction music? Good to hear it. Think about changing attacks/spells sounds aswell and then we will be fine. xD

This doesn't seem too hard a thing to do, coding wise. In fact, Total Annihilation offered similar functionality back in the day (once you started the game, you could swap out the disc for an audio CD, then assign specific tracks to ambient or battle themes); and Ridge Racer 6 (and, presumably, subsequent iterations) do have individual tracks as part of their DLC roster.