Should return:
Solas - I mean, obviously they have to finish this. Nobody has more "obviously going to return" status than Solas.
Dorian - He makes the most sense to return of any other companion right now. I would be fine with having him as a companion again, even.
Maevaris - Not really "return", I guess, since she hasn't been in a game yet, but she and Dorian are kind of a set, I think. Both are involved in the same plot. I think she might make the better companion option of the two, since we already had Dorian.
Calpernia - She always lives and she's an interesting character. She could be a good companion option with an interesting perspective.
Would like to see again/might make sense in certain context:
Morrigan - Some people are tired of her, but she's so wrapped up in important plot elements. She might have drank from the well, she's Mythal's daughter. I can't see just cutting that lose, honestly. I hope she comes back.
Merrill - What ever happened with her Eluvian? I'd really like to see what happened there and get her perspective on Solas and the truth about the Evanuris.
Sten - If we actually go into Par Vollen, then I assume we'll get a situation like Wrex vs the other guy in ME. He's a world leader now, so it makes sense we'd deal with him eventually.
Divine Victoria - Whoever it is, I think we'll probably see them again eventually. Depends on if we're dealing much with the south, though.
Vivienne - Even if she's not Divine, she's head of the Circles. I think we'll see her and whoever they choose to make head of the College of Enchanters eventually, but might not be DA4. I'd be surprised if there was no mention at all of what's going on with the southern mages.
Fenris - He was the most linked to Tevinter of past companions before DA:I and he has a unique perspective. I'd love to see him if we take an approach to empower slaves, or something of the like.
Zevran - I would love to see him in Antiva, if we ever go there. I think it's dependent, though. If we ever see the HoF and they romanced Zev, it would be nice to see them together, since supposedly they were looking for the cure together. Zev's relevance depends on whether there's a plot heavy on HoF or the Crows, though.
Isabela - I'd love to see her as an admiral. Possibly another that could be used if we ever go to Antiva. I would really love to see Antiva, just generally. It seems like such a cool place. We need a pirate heavy DA game, imo. It would be awesome.
Varric - I don't want to see him in a companion roll again, but it would be nice to see him in the capacity of Viscount eventually. I'd also like to eventually have a conclusion to the Starkhaven and Sebastian vs Kirkwall and Varric situation.
Sebastian - Personally, I'd love to be able to kill him, because I hate Sebastian. He's literally the only person in the series that I've ever felt that way about. If we ever go to the Freemarches, I could see having to pick between supporting him vs Varric, generally.
Shale - She was a really interesting character and I'd like to see if she ever accomplished her goal of becoming a dwarf again.
Probably not / not for a while / don't care -
Leliana / not Divine - It makes sense for Leliana to come back if she's Divine, but otherwise I wish we could back off her a bit. She's been really heavily covered in the games.
Alistair - If he's king (and for me he always is) then I expect to see him again, but I don't want anything related to Ferelden for a long time, if possible. I'm just so tired of Ferelden.
Iron Bull - I feel like he just has too many variables to handle well. Could be dead, could have been skipped for recruitment, could be with Dorian, but you could have romanced him or Dorian. Fenris was with Isabela, but they didn't keep them together, apparently. To use Iron Bull, I feel like they need to distance him from Dorian. They could have a possible line where Dorian mentions him in states where neither was romanced, but I don't see him appearing due to all the variables. They'd have to do several different versions to include him and also have the possibility that he's just dead.
Scout Harding - She's popular and probably will show up, but I don't actually care about her character. She just read as bland, to me. I don't really understand why people like her so much. I didn't even dislike her. I genuinely don't care if they put her in the game. I won't be disappointed if they do, because nothing about her rubbed me the wrong way. She just didn't interest me at all, either. Not particularly charismatic, no connections to greater plot threads or lore. She's just there. Given the choice, I wouldn't have her as a companion. I'd rather have people more connected to the plot and culture in Tevinter. "Random surface dwarf" just doesn't have much meat. Bianca would make a more interesting alternative for that, imo. She's more experienced, more knowledgeable, more connected. Harding is only connected to the Inquisitor, but we know everything the Inquisitor did, obviously.
Tallis - I hated her and MotA so much. There are plenty of other options for Qunari characters. Please let's just forget MotA ever happened.
Anyone from Awakening at this point - It's been too long, imo. Most people wouldn't know who they were now and I think they could easily find another character that would more smoothly fit in. I think characters like Velanna and Sigrun missed their window already.