I wouldn't be too surprised if none of the members of Dragon's Watch make an appearance (except for maybe small cameos at the beginning or the end). My prediction is that the commander is somehow completely snatched into the Amnytas region and the conflict that's going on, without any aid from their established allies.
Since Zojja seems to be returning (which I'm actually not particularly excited about, she's been absent for so long that I really don't have an attachment to her as a character anymore. Maybe that'll change during the upcoming story.), I'd like to see her interact with the former members of Destiny's Edge, talk about her feelings regarding Eir's death etc. And, although I don't see her returning with a very large role since we've spent so much time with her during the Elder Dragon saga, I'd like to see some interaction between Taimi and Zojja. IIRC, they've only ever interacted with each other off-screen, so it would be cool to see what their dynamic is like.
As far as other characters go, I think I'd like to see Livia return. I feel like she fits the theme of the expansion pack (ancient magic, powerful mages, seers?) very well. If Amnytas is a sort of reflection of Tyria below, we could potentially see some characters from GW1 return as reflections of their past selves. I feel like Livia could be a great addition to the team, if we do meet any familiar faces.