If you were to promote a minor character to greater relevance, who would it be?

Sticking purely to characters we have encountered in the games; Charade is one who springs to mind. She gives a link to Hawke and also Sera but can also is independent enough of them that it wouldn't seem out of place for her to be up north. I think she was linked to the Red Jennies in Tantervale in the War Table mission but that doesn't mean she could not have moved on into Tevinter from there. There is also a passage in WoT2 about her that is interesting:
"We find no account of Charade's birth name, a seemingly deliberate and recent absence of records. It suggests that she will be a noteworthy figure in the future, but for her new associations, not her lineage."
If the Red Jennies were going to establish a cell in Tevinter, Charade would fit the bill as the leader.

I'd like to see Feynriel return but unfortunately his outcomes are sufficiently diverse that it would probably preclude him from consideration.

So many characters wouldn't really have a reason to wind up in the northern part of Thedas. The beauty of Isabella was that as a ship's captain, she could turn up pretty much anywhere with a coast. Sabrae clan suddenly upping sticks and travelling overseas to the Freemarches always seemed a bit of a stretch to me, but since they did, it allowed for Merrill or any of them to appear in another location. There are not many other minor characters that would seem right to me.

However, some of the multi-player characters from DAI had interesting back stories, so I wouldn't mind giving them greater relevance in the future. My particular favourite is Cillian, the former Dalish who spent years in the wilderness at an ancient shrine to the Arcane Warriors, learning his craft. It says that when the breach appeared he knew that after years of living alone it was "time to return to the world and dedicate the skills he had learned to fighting for the greater good of all Thedosians." We sometimes referred to him for his learning on War Table missions and if he was serious in that declaration then I assume he would vehemently oppose Solas, so he would give the perspective of an elf who revered the traditions of the elves but was grounded in the modern world.

Another interesting person from multi-player was Tamar, the former Disciple of Andraste from Haven. Her biography says she never surrendered her own beliefs but joined the Inquisition forces as an alternative to prison. Being a Reaver she is a formidable warrior and, I should imagine, opposed to Solas. With the Inquisition being either disbanded or reduced to being the Divine's private army, I would think she would have taken the opportunity to recover her freedom from the organisation and go her own way in the world. I think having her travel to Tevinter and hearing her take on everything she finds there would be interesting, particularly as she is someone who has her own take on Andraste and the Maker but doesn't follow the teachings of either Chantry.

Or going back to DAO/Witch Hunt, how about Finn and Ariane? They were apparently still together subsequently when Finn returned to his parents' home to say goodbye. After that, so far as I am aware, they weren't seen again. So, they could be practically anywhere so why not up north? If the word was going out generally to the Dalish that something was happening in Arlathan Forest and they should head that way (which seems likely from Tevinter Nights), Ariane would probably at least want to investigate and might well take Finn with her.

Feynriel would be my first choice but he has to many possible outcomes in DA2. Another one i would choose is professor Kenric, i think it would be interesting to have a scholar for companion. 

Myself I would go with Shianni though her screen time was limited she perked my interest when she aided the warden investigating the plague (DAO) and aided him/her when the Darkspawn invaded the alienage, could throw in her brother as well he did try to help they could both be an asset they were there during the Blight and during the course of DAII and DAI gotten stronger and bettered trained.

Why should Bevin be a Lord of Fortune not just an adventurer with a sword?

Probably, but to be fair he is alive in all those outcomes, so it could be doable.

Eg.
We need a dreamer and Varric, our contact, is like "Well I know one but there's a problem.."
A. "he's tranquil" quest proceeds to cure Feynriel of tranquility.
B. "He's possessed" onward quest to hunt down the Dreamstalker and evict the demon.
C. "He's in Tevinter but he's otherwise occupied" quest follows to recruit Feynriel, complicated by him being busy. Like he's doing some magic research, or investigating a ruin, or fighting in an Tevinter/qun battle.

It would have to be a big enough role to be worth multiple recruitment quests (eg we need a dreamer to shield our minds or Solas will just kill us in our sleep), but probably better not to be a companion, as they have the most dialogue and would likely recquire more than one line of dialogue to have variants referencing things like whether he lived in the circle, with the dalish, and/or Tevinter aswell.

Of course whilst Dreamstalker Feynriel could have learned to use his powers from Torpor, and Tevinter Feynriel from one of many schools or magisters, Tranquil Feynriel presents a training problem they'd have to write their way out of...
Actually nevermind, since you'll need a spirit to touch his mind they could write it staying to teach/help him, ala Wynne and Faith. Easier than I thought.

Overall it would be way easier to just create a new Dreamer character, so it probably won't happen, but if they really wanted to..

I classed her as a major character, as she would be far more familiar to people than others we have mentioned and does play a major part in the Templar path, plus having her own short story.

I think we are likely to see him again seeing as he featured in Tevinter Nights. A lot depends on what they intend doing with the Qunari plot and what the allegiance is of the qunari lady who features in much of the concept art. If she is our Qunari contact, then we are unlikely to see much of Gatt but if she is a Vashoth or Tal-Vashoth, then he might feature as well.

Abelas would be a good candidate...we can just say in the event that Morrigan kills him that some link to Mythal or by her grace he survives. His ancient knowledge would be useful for contextual purposes; fulfilling the role of the mentor or wise archetypes. He would be Solas's opposite; but filling much the same role. Maybe his tie to Mythal would make him aware of Solas taking possession of her and he wants revenge. Just spitballing here.

Michel De Chevin. The Orlesian agent you can recruit in the snowy map in DAI.

Should have really been a companion in DAI

Didn't we have the option of killing him?

I've been pushing hard for this on the forum already, but I really do want to learn more about the current leader of the Grey Wardens in Weisshaupt, the First Warden.

I do expect them to bring the character in for DA4, especially when they discuss the relationship between Tevinter and the Wardens, but I'm curious to see who this First Warden really is.
Magister Sidereal?
The warden who killed Dumat and lived?
Another unknown figure?

The possibilities are endless.

Well Sandal. Main reason I like that goofball. Also I had a fanfic in mind that in involves HoF and Orlesian Warden meet up and encountering him and Sandal in his unique way helps them both on their respective missions and thus saving Thedas. Also it is not an complete idea but I will make room for Inquistor and Hawke.