First, if you're basing your assumptions off the game being released or not, game theorists won't have anything to do. They theorize game stories/features based on trailers, so I'm very confused on why the sentence says "the claims are being backed up by trailers" because I can't tell if you're pro-theory or anti-theory. It is your assumption, but basing it off the actual game isn't an assumption, it's a fact. That takes all the mystery away from a game theory.
To answer your question, I think having the Aether Foundation be evil is a very interesting plot twist. I was talking to my friend about the possible theory. To simplify my theory, I will have Lusamine be L, Li be Lillie, Gladion will be G.
The theory is the Aether Foundation is evil, and Li and G are both L's children. In the beginning, everything was going well. Li and G are assistants of their mother, L, and everything is going fine. But the story of Ultra Beasts starts floating in the air, and L is determined to harness the energy of them. Of course, L doesn't know exactly what they are or what they do, so she needs to obtain a Pokemon that might know exactly what they are. L thinks and thinks, and she finds out; she needs to harness the energy of Arceus. Of course, capturing Arceus is impossible for her, so she tries to create it herself. On the way to the creation of the Pokemon, a lot of Pokemon were hurt or killed. The Foundation believes it's for the best for the world, but Li and G don't think so. Li hates seeing Pokemon get hurt, while G fights to show dominance toward other people, however he cares a lot for his own Pokemon. He only wants to see them get stronger so he can defeat the Ultra Beasts in the future. Because of this, Li and G run away from the Foundation and their mother. But they both suffer from different morals and ideals, so they split off on their own. Li goes to the Pokemon Research, working for Professor Kakui. She learns a lot about Pokemon, so maybe one day she can prove her mother wrong; there is no need for brute strength or death to understand Ultra Beasts. G goes to Team Skull, where he hides until he can prove his dominance and connection with his Pokemon, that you don't need to kill Pokemon and forge a new one to defeat Ultra Beasts. G even takes one of the forgotten Pokemon, one of the failed experiments with him: Type: Null.
Of course, my theory has a lot of holes in it. It isn't backed up by solid proof, but neither is any other Pokemon Theory out there about Sun and Moon. But it would be interesting to see how it plays out.
