It's already so polished! Maybe you could add some new enemy types, like an enemy that switches lanes each time you hit it, or an enemy who's transparency fades in and out.

Maybe an ult in which shots are fired on all 3 sides at the same time, for like 3 seconds.

Honestly, the game is in such a good spot that I don't know a lot of ways to improve it.

Maybe surprise modifiers? For example, the lanes are now curved and requires a bit more attention to find the correct one to shoot

Another thing would be harder difficulty modes, with special settings like controlling multiple shapes instead of one for the people who love a challenge.

Achievements would also be a nice addition. " Clocksmith - Beat a level by only spinning clock-wise", " Pre Fire - Kill an enemy within 1 second of them spawning", or just milestone achievements for hitting certain levels. There's plenty of room to explore.