Obi-Wan should be in conflict with Yoda over whether or not to train Anakin and get involved in the Clone Wars. The OT implies that Yoda opposed the training of Anakin and getting involved in the Clone Wars.
Yellow eyes, looking like a chainsmoker, and Force Lightning should be exclusive to Palpatine. It’s not special when everyone has it.
With the exception of Palpatine and Vader, the Sith should be loyal to their masters and to their apprentices, not power-hungry backstabbers. This way Palpatine and Vader are unique characters who have a chance of changing the galaxy.
There shouldn’t be any references to the Sith existing before Palpatine. This way the big bad is clearly Palpatine, not some vague concept of “the Sith.”