Was there ever a plan to show Coruscant in the original trilogy? If so, why didn't they show it on screen in the end?
Was there a plan to include Coruscant in the original trilogy?
Sort of, but it wasn't called Coruscant at the time.
As Rogue Jedi alludes to in a comment on the question, the planet Coruscant didn't exist until Timothy Zahn's 1991 EU novel Heir to the Empire1; it was later added to what was then called G-Canon (what we now just call "Canon") with The Phantom Menace.
However, a proto-Coruscant called Had Abbadon was one of the main settings in a 1981 draft of Return of the Jedi:
The boundless heavens serve as a backdrop for the MAIN TITLE. A ROLL-UP crawls into infinity.
The Rebellion is doomed. Spies loyal to the Old Republic have reported several new armored space stations under construction by the Empire.
A desperate plan to attack the Dreaded Imperial capitol of Had Abbadon and destroy the Death Stars before they are completed has been put into effect.
Rebel commandos, led by Princess Leia, have made their way into the very heart of the Galactic Empire: as the first step toward the final battle for freedom....
Pan down to reveal the planet HAD ABBADON, capitol of the Galactic Empire. The gray planet’s surface is completely covered with cities and is shrouded in a sickly brown haze. Orbiting the polluted planet is a small, green, moon, a sparkling contrast to the foreboding sphere beyond.
Had Abbadon was later abandoned, with most of the plot ideas surrounding it getting rolled into Endor.
Also, in an early draft of what would eventually become A New Hope, we spend a fair bit of time on the Imperial Capital as Lucas envisioned it at the time: a planet called Alderaan:
A title card appears over a sea of billowing clouds on the gaseous planet of Alderaan:
Alderaan: Capital of the New Galactic Empire.
The towering white oxide clouds pass, revealing the Imperial city of Alderaan. The magnificent domed and gleaming city is perched, mushroom-like, on a tall spire, which disappears deep into the misty surface of the planet.
Alderaan of 1974 doesn't look a whole lot like the Coruscant we came to know and love, but as the Imperial Capital it's still an early concept of the same world.
Why didn't Coruscant appear at the end of Return of the Jedi?
Because the montage of celebration shots was added much later. Per Wookieepedia, those shots were added for the 1997 Special Edition release, and included a shot of Coruscant (which wouldn't be canonically called that until The Phantom Menace in 1999):
- At the end of the trilogy after the Empire is defeated, scenes of celebration on Endor, Bespin, Tatooine, and Coruscant are shown. The "Ewok Celebration" song (often referred to as "Yub Nub") during this sequence has been replaced with an entirely new John Williams composition, "Victory Celebration". A few quick shots of the Ewoks celebrating by bonfires and with different musical instruments (horns) have been added.
1 This isn't entirely true; the concept of an Imperial capital planet had existed before Zahn's book, though I'm not personally clear on how much Zahn added to the existing idea. Zahn himself was always adamant that he never invented Coruscant, he just named it; at the Celebration III convention, he said:
I did not invent the planet...George Lucas had invented the planetwide city a long time ago. When I was starting the Thrawn Trilogy, they told me to coordinate with the West End Games source material, and they had it listed as the Imperial Planet. Well nobody names a planet 'Imperial Planet,' so I thought it needed a name, so I picked the word that means glittering: 'Coruscant.'