I feel certain that any previously romancable companion will not be making a second appearance as a companion in future games, outside of perhaps as temporary one. There's too many variables involved there, and they haven't done so in the past. However, I can see a few of them returning as advisors in the next game, like Leliana did.
If the game does indeed take place in Tevinter, like so many believe it will, Dorian could easily be a Magister advisor, especially if you play a mage character.
Vivienne could probably show up as a companion again, possibly as an LI this time (as her paramour passes away in Inquisition).
If the game takes place in Tevinter and focuses on the next Qunari invasion, then Tallis will probably make an appearance, possibly as a Qunari spy and romance option.
I have to say that I am tempted to believe that Feynriel could be a possible companion... EXCEPT that there's too many variables surrounding him. He could be Tranquil, or possessed, or living in the Imperium learning to harness his abilities. I would love to have him as a companion, but there's too many possibilities to have him around. As a side note, however, wouldn't it be amazing if, as a mage in the next game, it turns out you are also a Dreamwalker? Then if he is available (not Tranquil or possessed), he can probably be an advisor or teach you an extra ability.
One person who I would love to have around is Finn (or Florian Phineas Horatio Aldebrant Esquire). He was one of the best parts in Witch-Hunt. Apparently since we've last seen him he's matured quite a bit, and done some more traveling with Ariane. It's also implied (by the replica necklace he wears which looks like the one ancient Tevinter priests wore) that he is very interested in ancient Tevinter, the Old Gods, and their history. He would fit in perfectly. I hope he's not too changed though- I would hate to have him be completely hardened and no longer adorkable at all.
Along with Finn, Ariane would be an interesting companion to have along. The two of them together would have quite a few fun conversations, I am sure. But knowing how BioWare works, they'd probably make it so that you could only recruit one of them, not both.
I'd very much like to have a Chasind or Avvar companion. They're always mentioned but so rarely seen.
I've pretty much resigned myself to having the next game be in Tevinter, but what I would really love is to be able to play a character who is Avvar, or Chasind. I love their cultures, their people, their religions- I just find everything about them to be fascinating. And since we had the Jaws of Hakkon DLC in Inquisition, it wouldn't be too out-of-the-blue to have an Avvar hero. After all the main villain of Inquisition first made his appearance in a DLC for Dragon Age II.
I know that there are a lot of people who still want Varric around, but I think his time is over. He's played a big role in two DA games, and it's clear that he's going nowhere fast. He's stuck in the past, and keeping him on would be pointless in my opinion, even as an advisor.