Vader vs. Sidious (See Details)
If Sidious did something to anger Vader, for example maybe he kills Padme in front of him, do you think Vader would be able to defeat him? Would something like that awaken his full potential so that he’d be able to kill Sidious?
I guess this pretty much happened already in RoTJ when Palpatine was going to kill Luke, but that wasn’t a true fight.