The role of Hawke and the DA2 party in DA4.

Exactly. I find absurd that Merrill is even considered to be character to join Solas. Who is next Fenris, Zevran or even Sera?!? 

Alright, I completely forgot about how much Merrill hates The Dread Wolf, the Satan of her religion. (It's been a year since I played DA2)

But let's rethink the prospect: how about Merrill following after Solas in order to stop him but then gets tricked by his agents and has to end working for Solas?
Now that's in character for her!

Her sister and her connection to the Blight are reasons why I could see her connected to Solas, since he hates the Blight and many theorize that's why he is doing what he is doing. So if he rids the world of Blight, thus saving her sister Serani, and promises to restore the elves to greatness she would join. Again the only hesitation being the fate of her non-elven friends (that's also why Ariane is a question mark due to her attachment to Finn). Fair about Sketch but I was only really referring to main companions (Zevran, Velanna, Merrill, Fenris, Tallis, Sera). 

I have no problems with those since I don't have to pretend to get to know them all over again and they didn't have their time in the spotlight yet.

That said, I can't think of any such character that I'd want in DA4, so I'm back at "ideally no one".

No interessed in Harding, Calpernia or an Adult Bevin as possible DA 4 companions?

Hawke is probably going to rest for the rest of their days... one way (in Kirkwall) or another (in the Fade). Though, if Hawke is alive and (as it seems) Varric hangs around in DA4, I wouldn't be surprised if he appointed Hawke Provisional Viscount of Kirkwall while he's out with us.

Varric seems to be returning... well, I love him, but I think I'd rather have him as some sort of advisor rather than occupying a companion slot. Also, I bet we'll see some "This Shit is Weird: The Inquisitor [X]'s Story" as codex entries

Fenris seems to make the most sense out of all the companions for the setting... thing is, the comics seem to deal with his personal problems already, so I don't know exactly what kind of personal plotline he should have... though, I'd be interested in seeing another lyrium-infused person in the team, just so we can explore the concept more in the game. It'd make for interesting gameplay too, I think.

Merrill - now she's the one I'd be most intrigued on seeing. Really, I'd just love to see a more... "mature" Merrill and hear her take on the whole Solas\Elvhen restoration\Evanuris plotline. Unfortunately I don't think she'll be back, but I'm sure we'll hear of her in some form.

Isabela was in one of the concept arts, if I remember correctly? I think we will see her, but I seriously doubt she'll be a companion. What if she's some sort of "advisor" for missions related to piracy\treasure-hunting and such?

Aveline - as Varric would say, Kirkwall would crumble before Aveline quit her job, so... probably close to 0 chances of ever seeing her again, unless we ever return to Kirkwall.

Anders is complicated... part of me would think that him finding refuge in Tevinter, living a life of a wanted terrorist in the South, is intriguing. Thing is, though, if Justice\Vengeance already had such an influence on him by the end of DA2, I fear years down the line we may find him as some sort of empty shell of himself, completely at the mercy of the spirit. If he's in the game, it's going to be a very small cameo, maybe in one or two sidequests or so.

Bethany\Carver - the only way I can see them in the game is if they're Grey Wardens and we find them at Weisshaupt. It'd work, but I still find it highly unlikely we'll see them. (part of me secretly wants to see a Warden Carver\Bethany having met the Hero of Ferelden and either gushing over them or being scared witless by them, but that's just my little senseless dream )

Sebastian - 99% sure we won't see him in the game... but it'd be interesting to hear more about what's he doing in Starkhaven.

I could see Fenris returning for a questline or smth, and maybe Admiral Isabela makes a cameo if we do any seafaring. But on the whole, I want new companions to get to know and not Hawke's ride or die crew lol.

Cameo's only please...just let them be happy damnit!

According to the comics, he’s doing very well at it.

A Warden Bethany/Carver would likely be at Weisshaupt. If we're going to see Wardens (not that I can understand why a Warden is needed against an ancient elven superracist), we would see them.

Aveline has no reason to be there.

Sebastian has no reason to be there.

Anders is always killed in my playthroughs, and that's really good.

Isabela could be there, as she operates in the Tevinter region. She's probably be part of the Armada.