Empire Strikes Back: planned or unplanned?

Honestly, while I have no doubt believing Lucas had a big binder of ideas and one day while making A New Hope looked at it saying "I could make 3/6/9 movies out those!" or that he originally wrote a new hope and told himself "I wish I could make a trilogy out of this", I do not believe the trilogy was "planned". There's plenty of apparent retcons and tweaks in the plot. Did Lucas had ESB's plot mapped out when he was filming ANH? I doubt it. He might have had small plot elements he later used in the sequels and prequels, but that's as far as he did.

Obi Wan wasn't even going to die until Guinness got bored.

I personally think he had all 6 films planned out (to vastly varying degrees) from the very beginning.

I just think on my own personal experiences with writing stories, and I can never include everything that is going through my head. Especially if he focused on writing just background notes, which is where we get all sorts of things like Luke Starkiller, the Journal of the Whills and Mace Windy.

There's an interview with Mark Hamill on You Tube from before ROTJ came out, where he tells the interviewer he had signed up for three movies before they started shooting the original Star Wars. At the very least, Lucas wanted to make more than one Star Wars. As to whether he actually had ideas for the sequels other than what he had gutted from his original too-long script, I doubt there was much there.

What about the prequels?

As I said, I don't doubt that Lucas was thinking about doing more things in the Star Wars universe, even if film was a modest hit and made a bit of money back. But from what I've read, the big epic saga (Tragedy of Darth Vader as he's called it) didn't really come into being until some time during or after the making of ESB. At one point it was said Star Wars might be a collection of films set in the universe: one about Obi-Wan, one about the Wookie planet, an adventure with Han and Chewie, etc.

It was the combination of Kasdan's darker script, Irvin Kershner's thoughtful direction, and Lucas' initial story decision of merging Vader and Father Skywalker into one character that altered the franchise from lighthearted space adventure series to grand story of betrayal, family, and redemption.

George eventually deciding no sequels but possible prequels down the road made sense, given that fans would more likely wait for the latter than the former and that with all the story elements tied up in ROTJ, he personally felt there wasn't any where else go afterwards.