If the mage companions weren't mages...

Huh. A bit of a hard question to visualize....

Dorian: He's a necromancer, but not because of any affinity with spirits or the philosophy behind death. Rather, he seems interested in the practical applications of spreading fear and destruction amongst his enemies. I would say a Reaver if it wasn't for how he seems better as a background fighter, but I guess an Assassin? It's practical, and gets the job done, but it may not give him the attention that he wants.

Vivienne: Given her personality, one would think she would be a duelist since it is said to have an emphasis on mastery, technique, and grabbing attention. But ironically (and to her derision since it gives her something in common with Blackwall), I think she'd be a Champion since she is both not afraid of the front lines and it fits the defender playstyle covered by the Knight Enchanter specialization.

Solas: Would be a bard. And I am not just saying that because of Tevinter Nights. But beyond the pragmatic approach of stealthily dispatching enemies, he is also adept at social maneuvering. Both in terms of striking against his enemies and rallying his allies.