I would love to have Scout Harding as a companion in DA4. And a decent dwarf LI would be great too. I don't think, they will bring back Varric a third time (I so would romance him), but Harding would be a great choice too.
And if the game really is set in Tevinter, I would love to see the return of Fenris and Dorian. It would make so much sense - they both have very strong opinions about Tevinter and it would be fun,seeing how Fenris will ruin every try of a diplomatic approach, if he is in the group and Dorian probably could give a lot of insights in his funny, snarky way. I would have voted for Felix instead of Dorian (although I really love Dorian) for variety's sake, but for that, we need a Mortalitasi in our group I guess (killing Felix off so soon was a waste of a good character imo)
And I really liked Krem and Lysas from DAI. I would love to see them both as companions in DA4. But I guess, that's not possible, because both of them can die? I'm not so sure about Krem (haven't finished my playthrough, but I got spoiled somewhat), but I guess, if you side with the templars, Lysas will be considered dead after 'IYHSB'. But since you never see a corpse (or did I miss it? I didn't like joining the templars, so went back to an earlier safe and recruited the mages), he might make an appearance.
As for LIs: So far, I always found at least one inetresting LI in the DA games. Some of might remember Baldurs Gate 2 with that whining knight Anomen as the only LI for a female character, but in that regard, Bioware did improve a lot
(I still consider BG2 as one of the best rpgs ever though).
But I have to agree with Apo, that a character should not be reduced to his LI value. In DAI for example I like to have Solas in my party, because he has a lot of interesting things to say, but I would never dream of romancing him. He's just a cool character with an interesting backstory and a lot of great interactions.