Thanks for the clarification. In general, I’m supportive of mods as long as they don’t redistribute the original game (and as long as it’s very clear that they’re unofficial mods, with proper credit given to the original game’s creators).
In the case of Hollow Knight and Celeste: Mods for those games require a legal copy of the base game to play. Mods for these games don’t redistribute the original game.
So, your mod would need to require that people get the base game from me first, before they can use your mod. If you did that, and also made it clear that what you’re releasing is a mod, then I think that’s probably fine.
Fair warning though: People spend time writing “Mod loaders” for games like Celeste/Hollow Knight to solve this problem, and it takes time and effort to do that.
To answer your last question: I do have plans to release the full game on Steam eventually (and the final version may cost money).