What if instead of Metallia & Beryl, Galaxia had been the one to destroy the Silver Millennium?
What if instead of Black Lady, Wiseman had brainwashed Usagi instead and she had turned evil?
What if the Black Moon Clan had won? What if Wiseman/Death Phantom had never come onto the scene? (Thinking 90's anime/musicals verse here)
What if Kaitou Ace lived?
What if instead of either Endymion or Princess Serenity it was Queen Serenity who Beryl had an unrequited love for?
What if there was a reverse Dark Kingdom arc? Where instead of the Shittenou being the ones to turn evil it was the Inners, and Mamoru and the Shittenou are the stars here (With Usagi being in the position of sort of a "Masked Moon?")
What if Mamoru was born 2-4 years earlier and was the exact same age as Usagi? (It cost literally nothing Naoko/Toei
)
Edit: Another
two three I thought of!
What if in the manga during the time Minako posed as the Princess, Mamoru mistook her for the Princess of his dreams and tried getting closer to her, thereby the two of them both falling for each other in the process? Alternatively...
What if during the day the Princess and Endymion met it was actually Venus doing another one of her decoy jobs, and while Endymion
thought he was falling for the Moon Princess, it was actually Venus instead?
What if during the Stars arc of the manga Usagi gave into despair and made the decision to destroy the Galaxy Cauldron like Cosmos originally wanted?