I just want Seijima to get more time to shine outside of prison. I love his combat style and his attitude, he just got the raw end of the deal in 5 getting stuck in prison for 2 chapters
Akiyama rocks as well though, definitely want to see him get more screen time as well since he had to share a chapter with Haruka so his side of the story wasn't as fleshed out as it could have been either.
Shinada was excellent in 5, on the replay his part was surprisingly the second best part for me (first being Kiryu's the action in his chapter was super solid). I just can't see how they really could do much else with Shinada, his story felt kinda complete in 5, but I'd love to be proven wrong.