You don’t need to change the level layouts, the color palettes, or the characters to make a great hack. Sometimes, all you need to do is make a minor tweak to completely change the gameplay and difficulty of the original game. Making it hilarious goes a long way, too. In Sonic 2 XL, that’s exactly what happens.
There have been hacks in the past where the rings are bad for you, but Sonic Retro members Ranger and Captain Bozo took that simple premise and ran with it. In Sonic 2 XL, the rings are replaced with onion rings. You need them to live, but if you eat too much of them, Sonic puts on pounds, inhibiting his running and jumping abilities. If you run around or hit a question-mark monitor, you lose weight. If you continue to eat more onion rings, you’ll have a heart attack and die. It makes the game so much harder as you try to collect as little rings as possible.
The edited sprites and animations are what makes this hack truly amazing. It would have been easy enough to simply alter speed and jumping, but having Sonic grow man-titties and get exhausted from walking short distances is incredible (and so damn funny).