Speaking of the ST as planned during ESB, the idea for ROTJ at that time most likely dealt with a Rebel raid on the Imperial base where clones were grown for the Empire's stormtrooper armies. From everything I can tell, of the three lead characters, only Luke made it out of this film alive: leading into the Sequel Trilogy immediately afterward where he meets his long-lost sister and falls in love with her. Han would undoubtedly sacrifice himself to destroy the clone facility, while Leia would probably be disintegrated by Boba Fett: an ironic echo of the fate of Alderaan. Luke would kill Vader, and the Imperial armies would lose their source of clones, but the price would be incredibly high. As for the clone planet itself, it's most likely a rain-soaked water planet: the original of Kamino in AOTC, as the Imperial palace on a lava world was the original of Mustfar. A fitting place for a clone laboratory, and also for the demise of Darth Vader, a symbolic Death figure. As the Orange Catholic Bible in Dune says, "From water does all life begin."