Maybe be able to have a simple pencil tool that allows the user to draw rough sketches in a special layer which people can pixel over. It could be similar to that of Pixaki.
How would this be different from making a regular layer for your sketch with the existing tools?
Or are you asking for the ability to draw on reference layers? Aseprite has reference layers like Pixaki, with the same ability to have a different pixel size than your art, but they can only be created from files, and cannot be drawn on (though they can be moved and scaled).
(Your wording is very vague and sounds like you’re just asking for the ability to use layers xP If you mean something more specific, please make that clear!)