Vader's Redemption in ROTS (had he became emperor)
If Anakin won the duel with Obi Wan, and then he killed the Emperor (without the suit he was much more powerful and also he could avoid force lightning), and Anakin became the Emperor himself, how could he have fullfilled the prophecy of the Chosen One and be redeemed, and bring balance to the Force?
Anakin's destiny was to bring balance to the Force. This situation makes it a LOT harder.
The Emperor trying to kill Luke because of Luke's refusal to turn to the darkside, was a catalyst for Anakin to be redeemed and be the Chosen One. The emperor had vader's family in danger, which allowed for the prophecy to be fullfilled and for anakin to turn good again.
Anakin killing the Emperor a little while after ROTS doesn't solve the problem, because you already have a Sith Lord-Anakin, around.
With Obi wan dead, and presumably Padme still dying, as well as a dead Emperor, only Yoda was left to confront Vader and try to save the twins from being raised as Siths. Remember, in the original trilogy, Obi Wan and Yoda expected Luke to be the Chosen One, as anakin was evil by that point.
Even if Yoda could rescue either Luke, Leia, or even both of them, how could they have redeemed their father, or possibly their evil sibling if Anakin took the other twin?
Maybe if Yoda took one twin and Vader took the other one, Emperor Vader would force lightning the good sibling and the evil sibling turns against their father to save their sibling, mirroring ROTJ. But if that was true, Anakin wouldn't be the Chosen One, because either Luke or Leia kills Anakin to save their Jedi sibling from him.
This situation becomes even more trickier if Anakin raised both Luke and Leia as Sith, redeeming him and making the prophecy happen.
Assuming Anakin was predestined to be the Chosen One, how would he have brought balance if he became the Emperor following ROTS?