I think Mei should get one. She became the Mizukage despite the fact that people with bloodline limits were being persecuted (see Haku). I mean she had TWO of them. I doubt the time eras were that far apart..
I would like to see more of when Tobirama was Hokage.
Rikodou as he was developing his art of ninshu ( ninjutsu )
Rikodou's sons as the fought thru the generations to become the Senju and Uchiha.
And of course, the fight between Rikodou Sennin and the Juubi.
Danzo would have an incredible backstory! Like how and when he got his Sharingan, how he got along with Hiruzen, how he got his Senju DNA, his fight against Shisui and taking his eye, when Itachi and Orochimaru worked under him for root. Danzo's backstory would be epic!!