I think there's something about whoever it is being feminine and Blupee-esque, and Rauru isn't feminine or Blupee-esque, but he is owl-like and Blupees are kind of owl-like, idk. However, the lead sage theory is interesting: they didn't need to get the sages to seal Ganon in BotW, but Yunobo, Teba, Sidon, Riju, Link, Zelda, and this being may end up being the seven sages of this game. Maybe, idk.
OOOOOOO, it could be SHEILA. Who's Sheila you might ask? Watch out I'm gonna tell you, she's Rauru's ex-wife, they got divorced because Rauru kept going into the Sacred Realm and not coming back for months and the kids needed a man to give them food, and so Sheila went out looking for money and found a Blupee and shot it with her bow and then the Lord of the Mountain was like, "Don't hurt my homies, bro" and turned her into a Blupee Owl Lady Goddess.
Sheila is going to give Zelda crazy God powers and she's gonna turn herself into a stone Zonai bird and fly Link around and Link is gonna go and find Midna who's been chilling in the clouds for 150,000 years and she'll be like, "Hey bro, I know Ganondorf is getting you down and destroyed the surface, but here's 7 elemental-themed temples you can go to and collect these capsule-tears from the Sheikah that cried before being violently sealed in the Twilight Realm during the Vaati resurgence." And she'll be like "also I found this cute ball of light, bring him with you, his name is Kyle."
And then Link is gonna go run around and find all the tears, and then they will un-turn Zelda from the bird and they, with their united power, will sing a pretty song and make Ganondorf realize that they are all existentially sad and they will all cry together and then Link will slowly stab Ganondorf through the chest because the Zonai hand tells him to, and then the Zonai hand will rip itself out of Link and be like IT WAS ME VAATI THE WHOLE TIME and then Sheila will sacrifice herself by falling in love with Vaati, and then Link and Zelda will say "screw this" and get on a boat and sail out across the sea, and then we'll pan down and Rauru will be standing on the top of Eventide Island watching them leave and we'll see him crying and then the game will be over, and we will realize that Kyle was Rauru the whole time and then WE will cry and rush out to buy the Oracle of Season remake.
Or it could just be Hylia like I suggest in the other thread.