It's easy to forget how amazing Sega's 3D Classics series was on 3DS. Not only did it give us amazing ports of some legendary arcade and home console releases, it added special features which made them even more unique—and Streets of Rage 2 was no exception.
The 3DS version added two new modes, Rage Relay and Casual. The former was arguably the most interesting, as it allowed the player to use all four characters during a single playthrough; when one character was defeated, another would step in.
That mode now lives on, thanks to a hack by PPI_Akiko. This adds the Rage Relay mode to the original Mega Drive / Genesis release but not the casual mode, which allowed players to defeat all enemies with a single hit.
Sadly, 3D Streets of Rage 2 is no longer available via the 3DS eShop, as Nintendo has switched off the digital storefront.
You can, of course, pick it up as part of the third Sega 3D Reprint Archives collection, which was released physically in Japan but is now quite expensive. Alternatively, you can hack your 3DS to play it.
You can download the patch here.