I"d have interest in this. How would we properly time it though?

I say yes to that, I'm curious as to how much time can be shaved off of each level.

Timer starts when selecting level, timer ends at entering Level door for standard stages, Final Hit for bosses, or "Vision Clear" for hoverboard and autoscroller levels. Video is required

At least that would be my idea of how it can be done.

Edit: Speaking of which, could you please add a 100% full category as well? "Required to start from New File. Timing starts on file select. Timing ends with final hit on 'All Visions Clear' splash screen. Video is required with submission."

A 100% catagory could be put on the IL leaderboards as well, requiering the collection of all moonstones in a level.

could we do something slimier for world speedruns?

Learning to run this game, world IL's would be very beneficial to practice

If you're learning this game, I'd suggest joining the Discord server (linked above), as it's more active than here and contain lots of information hardly accessible elsewhere.