Here's something I'd like to see:
A "practice" mode - the ability to practice a track starting from a checkpoint of your choice like you can do in the editor, but with any track. This would not only be helpful for difficult Ninja tracks, but to practice stringing sections of easier tracks together without starting an entire track over. For example if there's a shortcut you can't take from the immediately preceding checkpoint, it would be nice to be able to keep restarting two checkpoints before it to practice the transition without having to restart the track. It's incredibly frustrating when you can't practice a shortcut near the end of a track because failing to make it triggers a checkpoint on the main route below. Case in point:
The Alchemist by HEETSAUCE.
Anyone else think a feature like this would be helpful?