I’ll admit to not following theory threads too closely, but is it possible she’s Leia’s daughter from the start of the film, presented as a regular character? Not every Star Wars character has to be some long lost, hidden, unknown, secret, O.M.G. reveal.
The only theory I’ve seen about her anywhere is that she’s young Leia in a flashback, which she actually denied in an interview a couple months ago, so it’s not really a huge revelation that that isn’t true. I think she certainly could be Leia’s daughter. She’s the same age as Daisy Ridley though, so if she is then I think it’s unlikely that Rey is also. I also have this weird feeling that even if Billie Lourd is Leia’s kid she isn’t Han’s. If Han is in fact out of the picture, maybe Leia is/was involved with that Major Ematt guy who was in all the Journey to the Force Awakens books.