Honestly, I think it'll be Cole. Harding could go the Isabela route and become a full companion and Cole could be the returning companion, so we could end up getting both.
As for the others, I doubt it.
Vivienne could be
so I highly doubt she'll return.
Ditto for Cassandra, not to mention the fact that she's also a LI.
I highly doubt Leliana will show up at all. She's either running the Inquisition's info network or she's
Highly doubt it'll be Blackwall because his little secret's out in the open. Not to mention that he has multiple different possible fates.
Solas is an obvious no. Pretty self-explanatory if you've finished DA:I.
Varric has already been a returning companion once. And honestly, I feel that his character arc is pretty much done.
It's entirely possible that Dorian or IB could show up as the returning companion in DA4, but I doubt it because they're romanceable in DA:I. All the previous returning companions weren't romanceable save Anders, though he was only romanceable in DA2.
Honestly, I doubt Sera will return because 1) she's likely back doing Red Jenny stuff, and 2) she's romanceable in DA:I.
As for the other advisers, Josephine is an ambassador. Despite her past, she's the absolute last person I'd expect to be a companion. Well, unless BW decides to repeat the "innocent girl becomes a cynical pragmatist in charge of a large information network" trope that they've used for both Liara and Leliana.
Cullen would really be stretching it. I mean he was a Templar, so he knows how to fight, but he's also the leader of the Inquisition's armies, not to mention that he's also romanceable.
Overall, Cole's the most likely to return, imo.