Beth
Long suspected to be the woman at the center of this flashforward mystery is Beth (Susan Kelechi Watson), Randall's wife and Tess' mother. Kelechi Watson has kept her lips sealed. Given how much we're set to learn about Beth, plus her current storyline about her job loss and subsequent struggle, Beth still seems like the likely candidate for "her." But because it seems so obvious, it's probably not Beth.
Annie
We haven't seen now-adult Annie (Faithe Herman) just yet. Could Randall's other daughter be the mysterious person with whom they're visiting? Maybe, but there haven't really been hints about it.
Kate
The addition of old man Toby to this flashforward make Kate (Chrissy Metz) a solid candidate to be "her." Viewers recently saw young Tess (Eris Baker) and Kate bond in the Thanksgiving-themed episode of season three. Tess not only got her first period, but also came out to Kate and asked her aunt not to tell her parents about it.
Somebody else?
Tess' girlfriend or wife: This seems like a longshot, but the character did recently come out. However, an unseen and entirely new character wouldn't have the emotional resonance with the audience.
Kate and Toby's daughter: Again, like "her" being Tess' partner, this seems unlikely because viewers don't even know if Kate and Toby successfully have a baby.
Deja
Deja (Lyric Ross) was the prime suspect at the end of season two when scenes of her smashing a car windshield were spliced with the flashforward, but Deja's been doing well and managing her feelings.
Zoe
Tess is close with Zoe (Melanie Liburd), Beth's cousin, but she's been so tied to Kevin (Justin Hartley), that it doesn't make sense to have Randall and Toby tied to the visit.
Rebecca
Could we get Mandy Moore in a lot more old age makeup? Probably not, but…