Having an evil shanks makes sense for zoro.
Can beat evil shanks and be ready for mihawk
If this guy is a holy knight, then he’s realistically a villain for Killer or Bonney. idk.
But by the time rat’s twin enters the story, Zoro should be gearing to face Nusjuro.
I think Killer or Bonney could make sense. We don’t know who Bonney’s biological dad is, if it is Garling, then they’d be siblings.
As for Killer, we never knew why the Kid Pirates attacked the RHP to begin with. What if Shanks’s brother did something heinous, Kid thought it was shanks, and went after him for revenge? If the kid pirates survive, it would be a good way to tie them into the final war.