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." [sg-syndra]
I am sure the leak you are talking about was already confirmed to be fake.
However, I like the idea of having an "Anti-Star Guardian team", some evil champions controlling the creatures attacking the Star Guardians. Every magical girl show needs the big bad evil in the background.
Also, how I would approach it, it would be more like Puella Magi Madoka Magica. In this show, the girls are fighting monsters which are called "witches", later they find out that all witches have been magical girls before and turned into these by getting corrupted through the use of their magic and negative feelings.
I want the Anti-Star Guardians to be former Star Guardians, which "burnt out" over time (like stars do) and become evil through corruption. Also it would make sense for Syndras plot. She is still a Star Guardian, but perhaps she knows this secret and is embracing it, she is close to become an Anti-Star Guardian.
Great candidates for such a skin line would be:
