Streets of Rage 4 appears to be getting some DLC and that content is called Mr X Nightmare. This leak comes via SteamDB but nothing is known about the content. Of course, there can be theories though. Mr X is a series stalwart having been the main villain in the original Streets of Rage trilogy. Canonically, it is said the Mr X lost his life in a final showdown with the heroes, but there were alternate endings including Mr X becoming a brain in a jar.
When Streets of Rage 4 released DotEmu had stated that there would be more Streets of Rage 4 content coming out. The assumption has been that Estel, a boss in the game, would become a playable character. The DLC leak does not rule out the possibility and it could explore a potential link between her and Mr X if there is one. Nightmare could relate to the setting of the DLC or even its difficulty. Or both. It could even put Mr X in as playable character but against the heroes of Streets of Rage. Who knows?
In our review for Streets of Rage 4, I wrote: