The post about stickers in the Help forum got me thinking.
I think there is a way to manage stickers in Rebrickable as is, without development. The key is the alternate inventory feature.
Stickered parts would be inventoried as printed parts (Bricklink's term "decorated" would be better but that's cosmetic). The default inventory would stay as it is and have all parts unstickered. If someone has a set for which they want to keep track of stickered parts, they could submit the stickered parts then submit a second inventory where the stickered parts are included instead of the unstickered variations.
This would require no development and it would have 0 impact on people who don't care about stickers since the default inventory stays the same. Even if someone includes stickered parts in a MOC, since there is already a feature to ignore printed parts so the stickers could be ignored.
Obviously, a separate feature for prints and stickers would be ideal and this solution doesn't take into account cases where people only apply some stickers in a set but it would still be pretty good. The only impact I see is that the admins would have to manage the new parts and inventory submissions. But since these would be added on demand, not systematically for all sets in the database, it would probably not be massive.