The original draft of The Empire Strikes Back was written by sci-fi novelist and short story writer Leigh Brackett. She was hired by George Lucas to make a screenplay out of a rough story outline that he supplied.
Here is Brackett's original script, unedited by Lucas or others:
Tragically, Brackett died one month after completing this draft. The draft was heavily revised twice by Lucas himself and then given to Lawrence Kasdan, who changed the direction considerably.
In the original Brackett treatment of Lucas' story idea, there is a climactic lightsaber battle between Luke and Vader in an "orbital city" over "Besspin" (the genesis of the well-known Cloud City and Bespin).
What follows is a summary of pages 114-121 in the script:
- Vader squarely defeats Luke after Luke chooses not to use his anger (and hence the Dark Side) against Vader, although Luke felt brief instances of darkness in himself and is disturbed by them. Vader senses these instances.
- Then the scene turns surreal, with Vader towering over Luke and some kind of burning star field behind them as part of an elaborate Force illusion.
- Vader then plucks stars from the sky and throws them at Luke. Really.
He tosses a handful of stars over Luke's shadowy form, where they run over him like sparks.
- Vader impresses upon Luke the magnificent power of the Dark Side and beckons Luke to be re-trained in ways of the Dark Side. He claims that they can eventually "rule the galaxy together". Luke prefers to die. So they fight some more.
- Luke turns off his lightsaber at the moment that Vader is about to strike, causing Vader to fall forward. Luke then jumps down the shaft to the bottom of the city.
There is no mention of Vader being Luke's father and Luke's hand is not severed. Another large difference is that Han is not frozen in carbonite. Also in this draft, the Emperor remains simply a distant figurehead whose likeness is never revealed, as in A New Hope.
The film ends on a rather anticlimactic note, with Han and Lando leaving to convince someone by the name of "Ovan Marekal" to join the Rebellion and with Luke staying with Leia. (Han and Leia are now in love and have a tearful goodbye.)
As for the third film, there are the usual various claims that Lucas had story ideas fleshed out for three, or six, or nine, or twelves films, etc. etc. from day one. But I can find nothing concrete regarding his pre-ESB plans for the third film in the original trilogy. Once ESB was released in its final form, it of course nullified whatever tenuous plans he had originally.
(In contrast, there is a lot to say about the development of Return of the Jedi — or rather "Revenge of the Jedi" as it was originally titled — after ESB's release and hence after the Vader-Luke father-son storyline had already been established.)