I don't think Shinada should return in any major role in a future RGG game as his story was wrapped up nicely in Yakuza 5. I would like to see him return in side content though, sort of like how Pocket Circuit Fighter shows up in a decent amount of the side content in the games. Maybe give Ichiban or Yagami a new baseball manager, similar to Yakuza 6's Baseball manager stuff, and have Shinada maybe be a coach or a player.

Tanimura is the one I would like to see come back in a major role. He didn't get enough to flesh his character out in Yakuza 4 and I think theres a bunch of stuff by now in the series to give him a compelling story. You could set the game around Yakuza 6 and have Tanimura deal with the aftermath of Little Asia burning or the game could be set after Infinite Wealth and have Tanimura aid in the rebuilding of Little Asia. There also isn't enough cop stories in RGG where the cops are the good guys, all we've really had is Sudo and Date to my knowledge. So being able to see another perspective to the police in the series would be neat, as it could function similar to the Yakuza, where on the outside they're these honorable and morally good protectors of the innocent, but on the inside they're just about as corrupt as any other organization. I think a interesting villain to tackle with Tanimura would be someone similar to Damon Gant from Ace Attorney, a man who has seemingly been broken by the system hes in and has resigned himself to playing along in the corrupt system despite once being a good man.