From what I've seen, there is Any% for the game, which if I'm not mistaken is just every story mission back-to-back. But, has there been any consideration for a 100% run, being like all Gold Feathers etc?

I will be learning this game over time so if there is a 100% category, it might be something I'd be interested in running. If not, that's totally okay.

Often the way categories are added is because someone does a run for it and shows they're interested, so there's not a bunch of empty leaderboards

So if a category that "makes sense" isn't there, it could just be because no one has run it yet!

...doesn't answer anything specific about this game though of ourse

Just submitted a 100% run, its bad but it can be used to kick off the category.

I want to try a 100% run myself, there is still no official rules from what I can tell, so would the rules just be,

Finish all missions with gold feather collect all flyers and touch the yellow Q at the end

Or is there anything else you have to do, like buy all items?

When you view your save file it tells you your percent completion. All gold feathers and flyers are required, I believe you also have to touch the yellow Q and buy all items but I am not sure whether those count towards completion percentage.