It is quite difficult to ascertain just how many films George had in mind down the years 😉
Though if you back to the mid-late 1970’s era, before Star Wars '77 was a runaway success, George is on record stating he felt that Vader wasn’t strong enough as a main villain…
LUCAS: “I wouldn’t mind killing her off. (referring to Leia). The other thing we haven’t dealt with is Darth Vader. But Darth Vader, as we discovered in this picture, tends to be pushy; he’s not strong enough as the villain to hold the villain role. He doesn’t have the persona that you need. You really need a Cushing guy, a really slimy, ugly….”
LIPPINCOTT: “What if you unveiled him, unmasked him? Since he isn’t strong enough to hold up. Unmasked him and started building up a new villain who could continue into the next?”
LUCAS: “That’s an idea.”
^ from the Star Wars Splinter Of The Mind’s Eye Story Conference article at Cinetropolis.net. A transcribed conversation at the time of discussing a sequel to Star Wars - ‘Splinter Of The Minds Eye’ to not just be a novel - but a cheap-to-film sequel for Star Wars.
So, it’s 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 Lucas’ point of view that Vader not being up to much in the linked article above.
Note the date of the story conference: in mid-1976, when George Lucas was cutting together dailies of the original SW film with David Prowse’s broad Bristol accent speaking Vader’s lines instead of James Earl Jones’ menacing baritone. Lucas’ concerns about Vader being an ineffective villain would prove extraordinarily wide of the mark, in large part due to JEJ’s fantastic dub work.
In the 1975 second draft, Vader is indeed disposable: so much so that he dies kamikaze-crashing his starfighter into Han Solo’s pirate ship during the Death Star trench run. However, the film’s dialogue builds up another villain, Vader’s unseen boss Espaa Valorum, the “Master of the Bogan Force”, 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, he’s the guy who betrayed the Jedi in the climactic battle where Luke’s 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 Luke’s family history: in late 1975, Lucas told Alan Dean Foster that the second film would have the audience “learn who Darth Vader is”.
As for Grand Moff Tarkin, the character as such didn’t exist in the 1975 third draft: Vader was the sole major Imperial bad guy in the script. A “Grande Mouff Tarkin” 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 didn’t 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 film’s audience by removing the planned violence and nudity.)