I just beat this game, and I don't want it to ever end! This game has PLENTY of modding potential, from custom levels, graphics, mechanics, playable characters, gimmicks, etc. I hope it happens one day! A source code would be even better!

Well, it's not really what you want, but if you have a copy of PICO-8 you could make mods for the original/classic version of Celeste

We're looking into making an official level editor in the future, but I don't think we'll ever have official mod support. That said, I know a number of people are looking into doing game modding themselves.

I'm one of the devs of Everest, a modding engine for Celeste. Right now the only way to get mods is to ask on the Discord linked on the Everest page; 1.0.0 should have an official repository.