Don't Starve & Mario
Imagine the mix of Mario with Don't starve and here is why i think it would be interesting.
1. Still trying to save the princess with all the standard Mario powers.
2. Mario now has to survive along the way by finding food, building fires and shelters, leveling up, etc.
Instead of worlds containing levels Mario is now just dropped into the world and has to find his way to the Castle. He has to battle his way through the world and his enemies drop items to help him survive. but he can't just run through straight to the castle as he has to level up to become powerful enough. Leveling allows him to progressively handle new powers (ie. level 2 = standard mushroom, lvl 4 = fireball, etc).
I just feel it would give a great twist to the classic Mario!
Rocket league and Portal.
Taking place in the Rocket League universe the cars should be able to do everything they can right now but with the ability to shoot portals at various places. This will be an insanely challenging and incredibly fun. It will give a new challenge to the already most experienced players and a chance for new players to reach the top. For this to be viable the portals would have to be pretty small so that it will give you a huge advantage if you make it but it will aslo be a huge risk if you're not good enough to use them well.