I also think there’s potential for other characters to show up - Cad Bane could be a cool option, if he’s alive. After all, judging by what we’ve seen of that unfinished TCW arc, Bane - if alive - would likely be out to get him.
Indeed! Bane and Boba’s standoff from the cancelled TCW arcs is not considered as canon yet, so maybe it could be reworked into any of the newer projects as long as Filoni’s involved.
If I had to pick any cameos or returning characters they would be Bossk, Dengar or Embo.