I'd like to see different times of day and course themes (like mountains, snow, ocean, maybe even volcano?) implemented into the game, since it would add some extra, easy to change elements that make tracks more interesting.
Gameplay wise, I've seen many requests for boosters which I think would be awesome for making jumps etc more consistent on easier tracks! Something that I would like as well would be start positions that could be placed on top of tracks (similar to Geometry Dash, if you've seen that) which could be all deleted with a single button press for easier playtesting.
Finally i'd also suggest trees and flower planters as nice decorations/obstacles to again spice up courses!
Love this game so it would be really cool to see my suggestions implemented! :3