Yes I agree with it being pretty rigid the way I was heading with it. It definitely seems like games that are getting a good following are the ones where the players get to have more of a say on how it plays/sounds, etc.
I guess the way I program it can help with stopping people from making the song muddied up with kicks playing at a different tempo and sounding like a stampede of horses.
Just changing my towers to play sounds when placed has made it a bit more interesting (although I still have difficulty with interacting with specific instances of a sprite, but whatever).
I still really like the idea of the enemies being specific to instruments/sounds but the difficulty is making it a little more obvious of what you should use without having a bunch of text explaining them. I think the combination of the bright colors and specific enemies could work by having kicks being a bright red and the bullets they shoot are also red so the enemies that are the same color would be the ones you want to use the kick sounds against (or maybe using the inverse color of the tower?)
Its funny you mentioned Rez because I really like that game and the style it had. I definitely had the style of Rez in mind along with Tron when thinking about the color scheme/look of the game. I really like how simple but elegant Geometry Wars/Geometry Wars 2 looked (especially the warping of the background after bombs) and sort of tried to mimic it a bit but I'm waiting on a friend to make me some decent sprites.
I had actually came across the music TD game you mentioned after I initially came up with this idea. I tried it out and was really unimpressed with the looks and the gameplay of it like you mentioned. I was happy to see that was the most popular attempt at it because it was very far from what I wanted to accomplish.
Again thanks for all the ideas and suggestions! If you have anything else that randomly comes to mind definitely let me know, and when I finish this I will definitely make sure you get credit for helping with ideas :)