The prodigal son returns...

No one we know, it's a new character named Gwen. 

I mean, Chloe is the obvious answer since they're going through the trouble of bringing Nadia back. But if they don't rush them back together I could certainly see a pairing with Sarah too. Judging by Xander's reaction to Philip's return, he likely sees him as a potential rival and a brief Philip/Sarah pairing could heighten that, although I would expect Phloe and Xarah endgames. 

I think if you hear Nadia Bjorlin is coming back to the show, and Eric Martsolf is on canvas but no other obvious pairings, the mind naturally assumes they're doing Broe. But then if you near Nadia Bjorlin and Jay Johnson are coming back to the show at roughly the same time, suddenly, it's like "Oh, nevermind, I guess they're going Phloe." 

Theoretically Brady is now single and has no other obvious contract cast member pairing since Ericole are getting married and we know Sarah's not a real thing (it could become real, but I still think Xarah is what they have in mind long-term) so they could do him and Chloe, but I do think based on the Last Blast series the show has seemed to come down on the idea that Chloe's "true love" is Philip and not Brady, and Brady has been moved on to other things.

I think we know Kristen's back by fall. I don't think we know if she's ever coming back full-time or if her returns are just going to be drop-ins. But it's definitely Stacy.