There are many indications that Zoe and Syndra have history together. SG Xayah said Syndra and Zoe have history together. Zoe's trouble bubble when it lands has a similar face like Syndra's floating orbs. Both Zoe and Syndra use the exact same purple gem and clothes color. Zoe's right eye is corrupted shown as a purple glowing eye while Syndra left eye has a patch, which could possibly be corrupted too. This begs the questions, why is Syndra wearing an eye patch on her left eye. Did Syndra suffer an injury during battle or is she hiding her corruption by using that eye patch? Here on the official boards you can see that Syndra's right eye is glowing like Zoe's right eye on her official SG splash art, for reference I place it at the bottom of this post.
What is the history between Syndra and Zoe riot? Did Zoe fight Syndra? Are Zoe and Syndra using the same dark power? Was Syndra on that same ancient first star guardian team? Zoe's Star Guardian background and similarities leaves more question than answers about SG Syndra. Star Guardian Syndra's official lore is very accurate, her "history is cloaked in shadow." Riot should expand Star Guardian Syndra's lore possibly in the future. I would pay for a comic or even a book going into the details about Syndra, Zoe, and the ancient Star Guardians.