ARGH i have been reading this thread and find it an insult to fandom to use Videos games as evidence to support an opinion in a discussion of G canon.
VADER: Luke. You can destroy the Emperor. He has foreseen this. It is your destiny. Join me, and we can rule the galaxy as father and son.
Come with me. It's the only way.
Vader wanted Luke to join him in defeating the Emperor because Vader knew the Emperor was too powerful for him. Then after the Emperor's Deafeat they could rule the Empire.
Luke wanted to save Vader/Anakin, and in doing so they could defeat the Emporer, and bring peace and the Old republic back to the Galaxy. Tho Luke probably wanted to just save his dad and that was enough.
The Emperor on the other hand knew Luke was a threat. That is why he pitted Vader and Luke against each other. Luke could only win, by using the Dark side. And Luke Almost DID win, and succumb by a merest fraction. He only missed the Coup de Grace on Vader.
If Luke did the Coup de Grace The Emporer would have a new more likely more powerful apprentice. Otherwise, Vader would have won, and there would be one more less threat.
Vader more then likely could have ruled as a Tyrant in the Emperor's place. The only problem would be that Vader wouldn't have the Far-sighted vision that the Emporer had. I doubt Vader would have come up with the idea to lure the Rebels into a Trap with the Second Death star. Instead vader would likely Base-Delta-Zero any perceived uprising or rebel sympathizers.
If there are suppose to be troopers or royal guards defending the Emporer. How the heck was Luke able to drag Anakin's body to the shuttle without having to confront them? In the time it took Lando and wedge to travel to the Reactor core: Luke had to take the Turbolift down to the Death star surface, travel how ever many MILES to the Equatorial Trench (most likely an Express Turbolift for the Emporer's personal use) have an emotional Father sun talk and launch the shuttle.