Or instead of 100% custom rides, maybe more editing options for existing rides? For example, a ride that spins guests around could look like a complete different ride if you could change not only the color scheme, but the cars/seats the guests ride in, maybe get a few different choices of superstructure, and different "themes."
For example, a ride is a carousel when there are horses, but change it to cars, give it a new paint job and color scheme, and now it's the Thunderbolt. That sort of thing.
The rides are unfortunately the part of the game with the least variety. And I know that's understandable, but this game has been so innovative and fantastic that I think it can push the envelope further.