About the person who saved Luffy, given that it's supposed to be a shocking reveal, it means it's either someone very strong and important or someone that was supposed to be dead and who has some relevance to the arc.
Luffy is under the sea so it can't be someone with a DF.
It's also an ally (from Wano or that knows Luffy), otherwise there'd be no point in saving him.
With that said, we have a few options, like Rayleigh (who proposed his help to Oden in the past and he is Luffy's mentor), Garp (Luffy's grandfather and may want to end the Rocks era, only strong marine available), Toki (we still haven't learned much from her, we didn't see her die in the flashback).