What will be done regarding jelly cubes in custom levels? Obviously, the game can't just do what it normally does, because somebody could make a level with heaps of cubes and unlock everything in the game; however, they can't be completely blocked, because it would be fun (and necessary, if the ability to make any level is wanted) to have cubes in custom levels.
A half-solution would be to allow custom car designs to be given by custom levels, which would satisfy concerns regarding that variety of collectible, but what about jelly cubes? I mean, it would be unreasonable to give out, say, five different designs in one level, but what if there are five cool side-quests a player can perform within the stage?
To answer these questions, I suggest a separate jelly-cube-adjacent item: stars. Stars, in my vision, can be collected throughout a level, and are displayed alongside challenges, collectible collectedness indicators, secret exits, and Mormal Exits in the world information card (or outside the level if it is a standalone level). A level has a certain number of stars, and when a particular star is collected, that star is marked as complete. Ideally, this creates a system where the player is rewarded for finding little secrets around the level or performing interesting stunts, similarly to jelly cubes.
One shortcoming of this idea is that stars are a bit more "serious" than jelly cubes; an individual star can be found only once and is permanently registered as having been found, unlike jelly cubes, which respawn after the stage is reset or cleared. To this concern I have no rebuttal, seeing as this potential problem is deeply rooted in the way that this star system works. Perhaps the stars would respawn, and they could be collected and spent on custom antenna toppers which are provided alongside the level pack (so as to contain the original issue that this suggestion stems from). Further discussion is certainly needed for refinement of this idea.
Character limit?