I saw a leak awhile back and a few Anti-Star Guardians were mentioned.
Veigar, Zyra, and Syndra (Legendary).
Syndra already has a Star Guardian skin, so seeing this feels very unlikely.
If they did make this however, what would you rather have for her lore? Some cliche, "I was evil the whole time," or... "I split myself into good and evil for reasons."