A Game I'd Totally Play if It Didn't Not Exist

I had an idea for a game the other day that I think would be really fun: a rhythm-based brawler. I can trace the origin of this idea to two specific games I played recently: Samurai Champloo Sidetracked and Technic Beat. The Samurai Champloo game is similar to my idea in that the fighting techniques used interact with the soundtrack. However, the musical interaction is only superficial; you choose fighting styles by switching records in the soundtrack; each record changes the music as well as the combos your character can perform. Technic Beat inspired me from the other direction; it's actually a music game, but features a humanoid avatar who you can walk around in 2D space, rather than the completely abstract control methods present in other music games.

What I would like is a Final-Fight style side-scrolling game, with all the attacks constrained to the rhythm. I imagine this as a button sequence displayed over each enemy's head as the protagonist approaches; the player's task is to press the buttons in time with the soundtrack to successfully attack. Picking up weapons could change the soundtrack and the button sequences: something heavy like an oil drum could either cue a slower song or require a longer button sequence, for a slow but powerful attack.

The biggest problem I'm having with the idea is how to reconcile the rhythmic timing with an action game. If you approached someone between musical bars, for instance, would you both have to wait until the beat started? Maybe enemies could attack outside of rhythm, and you could block on-beat?

Anyway, now all I need to do is figure out how to make a game, and then make it. Really, no problem.