I found more information about this "microbiotic world" from Indie Wire
The two-time best director Oscar nominee confided his vision to none other than James Cameron, who interviewed Lucas for "Space," the second episode of his recent six-part AMC series "James Cameron's Story of Science Fiction." A transcript from their talk appears in the companion book of the same name, published by Insight Editions on May 18. In it, Lucas reveals that he would have turned his gaze from the cosmos to what can be viewed only under a microscope.
"[The next three 'Star Wars' films] were going to get into a microbiotic world," he told Cameron. "There's this world of creatures that operate differently than we do. I call them the Whills. And the Whills are the ones who actually control the universe. They feed off the Force."
Elsewhere in the conversation, Lucas admitted, "Everybody hated it in 'Phantom Menace' [when] we started talking about midi-chlorians." In terms of his storytelling, Lucas regarded individuals as "vehicles for the Whills to travel around in…And the conduit is the midi-chlorians. The midi-chlorians are the ones that communicate with the Whills. The Whills, in a general sense, they are the Force."
Lucas is confident that had he kept his company, the Whills-focused films "would have been done. Of course, a lot of the fans would have hated it, just like they did 'Phantom Menace' and everything, but at least the whole story from beginning to end would be told."
Here are more details from James Cameron's Story of Science Fiction
with George Lucas:
George Lucas: I keep telling people, look, we're not going to save the planet. Are you kidding? We don't have any power to save anything. But the planet's going to be OK. It'll look like Mars, but it's going to be fine. Who knows? Mars may have been like Earth and lost its atmosphere. . . . I still think they're going to find life on Mars. I'm absolutely convinced of it. There's going to be life all over the solar system.
Everybody hated it in Phantom Menance [when] we started to talk about midi-chlorians. There's a whole aspect of that movie that is about symbiotic relationships. To make you look and see that we aren't the boss. That there's an ecosystem here.
James Cameron: There's an ecosystem inside us called the microbiome that they're just learning about now.
George Lucas: [The next three Star Wars films] we're going to get into a microbiotic world. But there's this world of creatures that operate differently than we do. I call them the Whills, and the Whills are the ones who actually control the universe and feed off the Force.
James Cameron: You were creating a religion, George.
George Lucas: Back in the day, I used to say ultimately what this means is we were just cars, vehicles, for the Whills to travel around in . . . . We're vessels for them. And the conduit is the midi-chlorians. The midi-chlorians are the ones that communicate with the Whills. The Whills, in a general sense, they are the Force.
James Cameron: But you're putting detail and a facade of science around an idea that's pretty timeless, which is the spirit, the soul, heaven, causation . . . But in your world-building, you're going back to archetype, which is spirit, the godhead, all that sort of thing.
George Lucas: All the way back to -- with the Force and the Jedi and everything -- the whole concept how things happen was laid out entirely from [the beginning] to the end. But I never got to finish. I never got to tell people about it.
This is where The Journal of the Whills
came from way back during the draft period of Star Wars. So George wasn't kidding about knowing the entire story. I don't know, but the more I read of Lucas's idea the more it sounds intriguing. It is something original. I don't think I ever hear of a sci-fi fantasy film delving into such a thing. Perhaps Star Trek did so in a couple of episodes. But it would have been quite different for a Star Wars film. I assume the Skywalkers, specifically Anakin being conceived by the Force as implied, and by extension, his offspring would have had something to do with the Whills.
"Hard times create strong men. Strong men create good times. Good times create weak men. And, weak men create hard times." - Those Who Remain by G. Michael Hopf