I dont think its not someone we havent seen before, i just think its either someone we didnt think we be able to come back to life, or someone who has been introduced in the "background" so to speak. I was convinced it had to be Kabuto at first, but after Karin said she didn't recognize his chakra, that knocked that option out immediately. Someone had the theory that it could be one of the other zetzu pieces that we saw attached to the tree along with spiral zetzu...or it could be the sage himself or one of his boys. Those are my only thoughts on it as of now. Its def a dude, and Karin nor Hashi recognize him so hmm.....
The sage could be an Uzumaki, but i feel like this is a stretch. Idk why, i just don't see it being the sage was an Uzumaki.......mainly because Senju and Uchiha originated from his sons...Uzumaki could be a branch of course or an entirely seperate clan from Senju and Uchiha (which is what we have come to believe). Kaguya seems to have come from a time where no clans were really established, just people and heartache existed so for me personally, there just isnt even evidence to enough to say whether or not they were.