Star Wars Episode VI: Return of the Jedi brought George Lucas' original Star Wars saga to its perfect conclusion. From the perspective of the film's original audience, watching Return of the Jedi as the final installment of a Star Wars trilogy, that ending meant the Empire got defeated, the Rebels were victorious and Luke Skywalker's journey toward becoming a Jedi was complete. And when the prequels were released years later, and Anakin Skywalker's story got further explored, greater significance came to Return of the Jedi as the story that saw Anakin redeemed by his son, completing a story arc that spanned all six movies.
For George Lucas, of course, Star Wars was always the story of Anakin Skywalker and his redemption. Long before Lucas committed to making the prequels, he was already well aware of all his Star Wars characters' backstories prior to A New Hope. Vader's redemption at the hands of his son was always the fated conclusion to the story of Anakin Skywalker, as shaped by Lucas' interest in mythological storytelling that dealt with generational ties and particularly the father/son dynamic. Additionally, ever since the release of the Original Trilogy, it was established outside the films that Darth Vader gained his injuries in a volcanic pit, which would ultimately become the volcanic planet of Mustafar. But in early drafts of Return of the Jedi, Vader's fiery origins would have gotten echoed in the moment of his redemption.
As early as 1978, Star Wars Monthly Issue #2 reported that Darth Vader "fell into a volcanic pit" during a duel with Obi-Wan Kenobi. Later, in 1983's novelization of Return of the Jedi by James Kahn, Obi-Wan tells Luke that his "...father fell into a molten pit. When your father clawed his way out of that fiery pool, the change had been burned into him forever -- he was Darth Vader, without a trace of Anakin Skywalker." That means Darth Vader's backstory was determined early on in the Star Wars saga's development, and, originally, the character's final moments would also have occurred in a volcanic environment.
The original plans for Return of the Jedi included not one but two new Death Stars under construction. The forest moon that later became Endor was still there, but it was a sanctuary moon over the planet Had Abbadon - a sprawling city-planet at the center of the Imperial government. Had Abbadon would eventually become Coruscant but never made it into Return of the Jedi due to the budgetary and logistic constraints of creating such a world on screen. Thus, the film's final confrontation between Luke, Darth Vader and the Emperor moved to the second Death Star. Before Had Abbadon got cut, however, the scene played out in the Emperor's throne room deep beneath the Imperial Palace on the city-planet. And concept art by Ralph McQuarrie shows the Emperor's throne situated over a lake of lava.
Having Darth Vader's final moments play out over a volcanic pool, echoing the one in which Anakin got immolated, would have seen Vader's birth and death occur in the lake of fire. The Return of the Jedi novelization implies that Anakin's transformation into Vader was not truly complete until he suffered the burns that left him dependent on his suit, and that is a reading that holds up. While Anakin fell to the dark side before his duel with Obi-Wan on Mustafar in Revenge of the Sith, it was the moment he got sealed in his terrifying armor that Darth Vader was truly born.
Had Anakin's return at the end of Return of the Jedi occurred in the volcanic surroundings of Had Abbadon's throne room, it would have been as though he had re-emerged from the same destructive flames into which he had vanished decades prior. Where Obi-Wan had failed to save his friend and instead watched him meet his demise, Luke would have saved his father from that same fate and instead brought Anakin back from his supposed death. The confrontation in the Emperor's Death Star throne room is an iconic Star Wars scene, but Return of the Jedi's original molten pits would have ended Darth Vader's story where it began, more powerfully affirming Anakin's redemption as the fate for which he was always destined.