It is quite difficult to ascertain just how many films George had in mind down the years š
Though if you back to the mid-late 1970ās era, before Star Wars '77 was a runaway success, George is on record stating he felt that Vader wasnāt strong enough as a main villainā¦
ĀLUCAS: āI wouldnāt mind killing her off. (referring to Leia). The other thing we havenāt dealt with is Darth Vader. But Darth Vader, as we discovered in this picture, tends to be pushy; heās not strong enough as the villain to hold the villain role. He doesnāt have the persona that you need. You really need a Cushing guy, a really slimy, uglyā¦.ā
LIPPINCOTT: āWhat if you unveiled him, unmasked him? Since he isnāt strong enough to hold up. Unmasked him and started building up a new villain who could continue into the next?ā
LUCAS: āThatās an idea.ā
Ā^ from the Star Wars Splinter Of The Mindās Eye Story Conference article at Cinetropolis.net. A transcribed conversation at the time of discussing a sequel to Star Wars - āSplinter Of The Minds Eyeā to not just be a novel - but a cheap-to-film sequel for Star Wars.
ĀSo, itās a possibility that George had thoughts on other main villains lined up for Vader to work under in the continuing movies somewhere along the line - which could well have been Imperial Moffs and the like. There is a fair bit more on Lucasā point of view that Vader not being up to much in the linked article above.
Ā
Note the date of the story conference: in mid-1976, when George Lucas was cutting together dailies of the original SW film with David Prowseās broad Bristol accent speaking Vaderās lines instead of James Earl Jonesā menacing baritone. Lucasā concerns about Vader being an ineffective villain would prove extraordinarily wide of the mark, in large part due to JEJās fantastic dub work.
In the 1975 second draft, Vader is indeed disposable: so much so that he dies kamikaze-crashing his starfighter into Han Soloās pirate ship during the Death Star trench run. However, the filmās dialogue builds up another villain, Vaderās unseen boss Espaa Valorum, the āMaster of the Bogan Forceā, who would evidently take over his role in any sequels.
But by the 1975 third draft, Vader is obviously meant to stick around for a while; as Ben Kenobi tells Luke, heās the guy who betrayed the Jedi in the climactic battle where Lukeās father died and the Jedi Order fell. And this more deadly Vader now survives the Death Star fight to stick around for future films. Even at this stage, Lucas was apparently planning some sort of secret identity for Vader, with a familial connection to Lukeās family history: in late 1975, Lucas told Alan Dean Foster that the second film would have the audience ālearn who Darth Vader isā.
As for Grand Moff Tarkin, the character as such didnāt exist in the 1975 third draft: Vader was the sole major Imperial bad guy in the script. A āGrande Mouff Tarkinā is one of the Rebel leaders, but his title appears to suggest some sort of priest, akin to Friar Tuck of Robin Hood legends.
The character of Tarkin as we know him was added to the January 1976 fourth draft, as a proxy of sorts for the Emperor, whose death would allow for a satisfying conclusion if (as Lucas was beginning to fear at that time) SW didnāt make enough money to allow for full-fledged film sequels. (That this was becoming a concern for Lucas was evidenced by his turning to Alan Dean Foster to write SOTME as a concept that could be recycled into a low-budget TV movie, and doing other things like broadening the filmās audience by removing the planned violence and nudity.)