If I ever go back to update this it's likely I'll just rebuild it from scratch due to the amount of bad/old code in the game. The main reason for the lack of features in the game right now is the way the editor and saving system works, which I would also have to update.
Maybe I'll get around to that one day, and if I do most of these wouldn't be hard to implement. Also, what do you mean by "custom terrain"?
Like a menu to make new terrain blocks to use for the level, and the setup for it could look like this:
Edit: Took 20 minutes to make that in MS Paint
This would technically be possible within the lmitations of the engine I'm using, but:
1 - The entire space for the level is already taken up. This could be worked around using compression, but saving custom tiles would still be difficult.
2 - Creating a tile isn't as simple as it seems, 16 different variations would be needed to be made in order to adapt to any placement (which, again, would take up a lot of space in the level file).
3 - I feel like being able to create your own tiles makes it feel less like a Celeste editor, and more of just a platformer engine.
I would like to add more than just the default ice tiles though.