I was thinking just have the Shinobi uniform as an unlockable and that's it. Don't even need to refer to Jiro or Joe. It could be called "Shinobi unform" and say it's what a legendary ninja from the Oboro Clan wear or something.
It would be cool ... if they can at least got permission from who had ever worked on Shinobi or their original creator.
Jordi Asensio from Guard Crush (one of Streets of Rage 4 developers) explained how hard it is to get license to have Shinobi as playable character for SoR4 and explained how Sega is a huge company made of multiple smaller entities, and most of that smaller entities couldn't gave their blessing since they never worked in Shinobi before.
It all depends on who the character is related to inside Sega, and a lot of other things, so it's far from being a simple matter to include another franchise in the game.
They got rejected in the end while before Demiurge could manage to get permission to use them in SEGA Heroes.
![](https://i.imgur.com/cFBQGJ5.png)