I really hope she comes back in a DLC as a bad guy boss. In the lore from the Taken King calcified fragments, there is mention of Taox. Taox is the same species as Oryx and his sisters and has moved with the Traveler, running from Oryx for millennia. Also, according to the lore, once the god version of Oryx is slain, his power can be claimed by his vanquisher. Eris didn't slay him directly, but perhaps she used the guardians and the light as tools to kill him, since she is the only one who knows of this power being up for grabs. Plus, she was the only survivor of the first fireteam that attacked Crota, when she came back she was changed, not to mention the third eye which is a trait of the species that Oryx is.