You could do a password menu system, where each ending gives a password that can be input to view it again. That way the player can still view the endings they've earned while still needing the work for the ones they haven't gotten yet.
Aaw yeah that sounds great, though a bit more work than just having some secret spot to click on or other specific input. But I could see a password system working on like a multilevel platformer game for level select too, to have the system serve another purpose.
That would definitely work well in a stage-based platformer, especially if there are multiple scenes per stage; of course, it would also serve very nicely as a level select system as well.
