best bet is either general type of rogue, or spec a mage for support role I'd say. a DW Rogue spec'd for Assassin will be the way to go, thanks to heavy utilization of Stealth and the immense one-hit damage it can do. i run DW right now, and spec'd so I can stealth and twin fangs for a one hit, which instantly resets stealth, so rinse and repeat on the ranged enemies while my party (Bull, Dorian and Blackwall) focus down the hardier enemies. you could try tempest or artificer, but a tempest is very up close and easy to kill it seems, plus why would you run an artificer DW. Archery is also good, if not better, because artificer makes more sense here allowing you to have more control of the area in addition to keeping ranged during the fight. Also, it may be tricky but off the top of my head visualizing the assassin-friendly abilities and passives, an assassin archer is also very viable, you just have to stealth behind the enemies and get off that first shot on the biggest enemy, then hang back and take out the ranged ones as the rest of the party goes to work.
for the mage, anything is possible obviously, but support seems ideal because if you are controlling the support, you cant do damage but you control the when and where of the buffs and debuffs. personally the friendly AI for the mages seems kinda inconclusive, not sure about it too much but Dorian loves to not use his buffs unless i make him.
also, just for extra, i would shy away from Solas and Sera for nightmare. Tempest on a rogue archer seems like a waste and not very ideal for use on harder difficulties, but i havent used it and its new to the games so if im wrong bite my head off for it. for Solas, his specialization seems subpar to the others. I use Dorian and he is constantly stealing kills so we always have a spirit up, and Vivienne's specialization, while not ideal for nightmare, seems like its possible to make her a powerhouse. Varric+artificer is definitely a good pull, Cassandra has the magic dispels which when used on enemies spawning in from fade rifts can kill them before them fully spawn, and blackwall has the badass tank abilities in his specialization. Iron Bull is 50/50. I use him and he wrecks face well enough, but on harder difficulties I can see him being a bit of a glass cannon. I cant say much on cole since im a DW Rogue spec'd for assassin so I never use him, so if you like him go for it.
keep in mind I have yet to start nightmare because I have final exams for college coming up so I can't be playing my PS4 much, but this should hold up I would expect. if not tell me and thisll be gone