For my Nightmare run which will include the DLC will be:
Blackwall - Tank Vanguard/Champion Inquisitor - Archer Artificer (extremely OP and a tier 3 focus special can down dragons in 5 seconds) Vivienne - Barrier/Knight Enchanter Cassandra - Off Tank - S/S/Templar
Cassandra will be the only character I may swap out for either an extra range Archer like Sera or even Varric (another Artificer), I personally hate Solas so never use him, Dorian is good as well for the Necromancer tree. So far it's worked great with very, very little deaths or even potion consumption. I've just finished the Empress part of the story and only switched out characters for their specific quests so far tbh.