It doesn’t sound like much, but the new characters are actually not dissonant and look organic.
Modders Savok and PPI_Akiko released a hack of the original Streets Of Rage 2 image, replacing all characters and bosses with TMNT: Shredder’s Revenge heroes. Teenage Mutant Ninja Turtles: Shredder’s Re-Revenge features several “creative changes”, including a change in some of the sounds, as well as a new story and “Turbo Mode”.
The hacked image is available at this link. It’ll need an emulator to run.