I know most people think that Annabeth is the 7th but I'm unsure but I came up with a theory just now. What if the 7th member already died somehow? Like Hazel? She died but it happened by chance that she came back. And Frank? Hera told his mom and grandma about it and left it up to them to take it out of the fire, therefore he could have easily died if they weren't fast enough. What if Hera took a chance like that with another hero who was meant to be the 7th but he/she somehow ended up diying anyway. That would suck though because that means it would be impossible to defeat the Giants but..... If I were writting it that way I'd make that Demigod be reborn into another life and then become one of the 7. That'd be cool right? Like what if Its Bianca? Nico said she'd been reborn, right? And even if this Idea isn't relevant to anything that will happen in the story, could it be possible that we'll see Bianca again?
And If Annabeth was apart of the 7 wouldn't we have gotten her POV in The Lost Hero?