Darth Bane, at least his tomb and force ghost have appeared in season 5 of the clone wars. Yoda travels to Moriban ( cause Korriban sounded too much like Corusant) and discovered the valley of the Sith and Bane’s tomb.
C-3P0 would need to be Resistance with the Red arm and also be titled Espionage Droid and direct other droids to attack for him. Again, canon. In the new Disney authored novels and Marvel comics.