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.