Good question.
The idea of Vader having a dark Force master is one that goes back a ways; in the 1975 second draft Vader’s Dark Side master is Prince Espaa Valorum, the “Master of the Bogan Force”, an offscreen character who is evidently a shadow power behind the Emperor’s throne.
In the 1975 third draft and beyond, the Emperor was still seen as a warlord/tyrant/corrupt politician figure rather than a Sith sorcerer. But George Lucas was evidently playing with the idea of Vader taking orders from some sort of Dark Side boss figure, whether corporeal or not - perhaps not even part of this plane of existence. A mystery for future sequels, perhaps.
By the time of ESB, the two concepts were merged to some degree, with Palpatine becoming an evil counterpart of Yoda, as Vader is to Obi-Wan.
Another interesting passage is this one from Lucas’ 1977 story conference with Leigh Brackett, in JW Rinzler’s Making of ESB:
“We have to give Vader another environment, either another Death Star–type Imperial City or some kind of cave. Might be nice to give Vader a little castle on a rock in the middle of the ocean. One way to see him would be in a tall, dark tower, very narrow in a lava flow, dark, red, and burning, almost like hell. He’d be up in the tower with his gremlin, goblin type gargoyles surrounding him. His pets. Vader walks down the hall — these long, narrow, steel corridors, very gray — and he goes into a gray room. It’s all steel and there at the end of the room on a throne is a gray, macabre, cold steel box and it’s the Emperor. The Emperor tells Vader to get Luke — he is the last of the Jedi and must be stopped. Vader is saying he’s not a Jedi yet. Vader could talk to the Emperor with a viewscreen à la Flash Gordon.”
Which suggests the Emperor as some sort of person contained inside a life-support cubicle - like Guild Navigators in Frank Herbert’s Dune series, or Emperor Huon in Michael Moorcock’s Runestaff series. Or even a computer ruling the Empire in unfeeling machine fashion, picking up on the motifs in THX 1138.
Even during ROTJ, Phil Tippett noted that the makeup he designed for the Emperor was meant to suggest a strangely deformed cranium that was splitting into two, and that (quoting Rinzler’s Making of ROTJ) Palpatine was supposed to be “ancient, not old”, specifically “a Methuselah figure kept alive and intact by some unknown magic”.