What I ended up doing was keeping half note movement but allowing her to move diagonally but the player is still restricted to 4 directions. And then (spoiler) for the final key she moves to quarter note movement. Even that was almost too much so your analysis was spot on. I love your idea of adding more enemies, almost could do it Silent Hill style while still having the "main" enemy. And subsequently more genre study would be important I agree, the main rhythm grid movement ones I found are CotND, Cadence of Hyrule, and then this indie one called Steampunk Pirates. Patapon also might be good inspiration here. If there's any others you happen to remember let me know!