One of my Hawkes romanced Fenris and I thought it was a good relationship (Hawke was helpful/diplomatic, and only rivalmanced him because she honestly believed his anger was harmful to him)
In my other playthroughs, Fenris and Isabela hooking up didn't bother me at all. I liked that touch, honestly, and I wouldn't mind seeing something like that again in Inquisition. It's one thing for a love interest to actively betray you while in a romance (like Jacob) and something else entirely for them to find comfort and companionship with one of the other people they travel and work closely with when the player is not offering it.
At the risk of being murdered in my sleep... I'll go so far as to say that I'd like to see Cassandra and Regalyan and Varric and Bianca reunite in Inquisition - but rekindling a romance should only be possible if the Inquisitor hasn't called dibs yet. Perhaps it could work like Aveline's courtship quest, and if the player so desires, they can change their mind at some point during the quest and confess their feelings to Cassandra/Varric, who will be surprised, but happily reciprocate and let Regalyan/Bianca down easy while preserving their friendship.
In Fable 3, the hero's girlfriend/boyfriend either dies (by your order, albiet coerced) or you have to be a jerk and steal them away from a nice guy/girl they met after being separated from you.
If that were to happen in real life I would probably swallow my feelings, put on a smile and wish my beloved a long and happy life with their new love, because I am a wimp. In a video game, though? I steal Elliot back every time! And I'm not a fan of how that's considered "evil," either. Pursuing your own happiness isn't wrong.