I'm making a platformer and the idea was that when you beat a level you started from the end and had to beat it backwards. (Something would change like new hazards or a new ability) Does this fit the theme and is it an okay idea?
Yeah, I think that fits the theme. When you reach The End , It changes and so it is a new beginning. If you want then you can try turning the level upside down or make the character have some new way of completing that level. But keep in mind the time of the jam so that you don't fall into feature creep or scope creep.