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.