Should take Sera with Blackwall. They have some really great banter.
Banter-wise, there are definitely "intended" combinations.
If Blackwall is your tank, Sera. They develop sort of a "stepfather/big brother - stepdaughter/younger sister" relationship, however one may perceive the age difference. Sera and Vivienne absolutely resent one another as you may imagine, but in a very hilarious way mostly.
If Cassandra is your tank, Varric. It's quite entertaining how he keeps pushing her buttons; and also relating to past events from time to time. As others have stated though, since Varric does not become your sidekick like DA2, he does indeed lose part of his charm in a way. However, Varric even manages to win over Vivienne because of his charms and way with words, which does have its very own appeal.
Sera is a beast with Tempest, both offensively and defensively (Lightning Flask makes her also my team's medic); but Varric with Opportunity Knocks has access to one of the most powerful Passive Skills in the game. Both of their Focus skills deal insane damage.