I forgot to add...after Darth Vader had thrown Palpatine down the shaft in the Death Star, Vader rose up and told Luke to join him as his Sith apprentice since the Emperor was dead?
That would have been a very interesting twist in that Darth Vader's love for his son was his motivation to become the next Sith master.
Luke may have been Vader's prisoner as he was still hurt from the Force Lightning.
Obi-Wan and Yoda's ghosts would have contacted Leia and told her that she has to train to become a Jedi as back-up. Or maybe their ghosts could have appeared to Luke and Vader.
Episode 7 would have been a continuation from ROTJ--the Empire is in disarray and we see the Imperial Dignitaries and Darth Vader ruling the remaining factions of the Empire. The galaxy is quite large so there would have had to be another Imperial fleet out there and other worlds to explore.
Perhaps Luke would join the Darkside just to defeat it and Vader.