Instead of being chopped up and fried on Mustafar, Vader kills Obi Wan and makes his way back to Coruscant to overthrow Palpatine and take control of the newly formed Empire. He has time to rest up and prepare fully. Is he ready to defeat Sidious and take over as dark lord? All out in the same location Mace fought Sidious.
I wonder what the effect of killing Obi-Wan would have on Vader. Would it make him stronger, strong enough to kill Palpatine, or would he be weakened like Kylo was?
Well, that aside, I think Palpatine'd still have an edge over him. However... If Padme's death lead him to the same realization it did after his surgery, the resulting bloodrage would probably obliterate Sidious.