They have graphics for a lot of other characters. Examples include:

Dosu

Zaku

Kin

Chidori Sasuke

Kotetsu and Izumo

Genma

Karui nevermind, she's obtainable in CN

Tiger Mizuki

Frog Suit Konan

Edo Tensei Jirobo

Akatsuki Cloak Suigetsu & Jugo

Hayate Gekko

Kid Jiraiya

And more...

They could theoretically make any of these into a character at any given time. It's all up to them tbh.

I personally have been pushing for them to add Dosu since the beginning of the game's lifespan.