I recall an episode where Worf tells a tale that once upon a time in not-so-recent history, the Klingons sent an entire fleet to conquer the Breen homeworld, and the Klingon fleet was never heard from again.
From that point forward I was expecting the reveal to be that the Breen were actually differently-motivated Klingons with their own offshoot culture (sort of like the Romulans are for the Vulcans). Something like, victory at all costs and we'll gladly shoot you with your back turned, plus we run away from fights when the odds are bad (honour be damned). Honestly, the story would have practically written itself.
I really wanted an episode where Sisko is captured by the Breen at some remote location, and they mind-sift him and find out some "valuable intelligence" that could tip the balance in the Dominion War. The episode culminates in Sisko having some sort of personal confrontation with the Breen leader, in which he or she (plus one or two of the guards or aides) takes off their mask, the Klingon face is the sudden reveal, and the dialogue goes something like this:
SISKO: You're Klingons!
BREEN: Yes, Captain. We are descended from warriors of the Second Empire, we conquered the Breen centuries ago.
BREEN LEADER BRANDISHES [BREEN DISRUPTOR PISTOL]
BREEN: Unfortunately you won't be reporting this discovery to your superiors.
SISKO: I've never met Klingons before who murder unarmed civilians, destroy helpless ships, and hide their faces in battle. What would Kahless think?
BREEN: Kahless was a fool! His ideology, his sick obsession with honour beyond reason, has held Qo'noS back for generations. Klingons are warriors by blood, Captain. And as I'm sure you will agree, there are no rules in war. You created false evidence of a Dominion plot against the Romulans, to manipulate them into joining the war. You may have destroyed our transceiver array, but as soon as we return to Dominion space, your deception will be revealed, and your alliance will crumble.
[BOTH REACT TO SOUND OF DISTANT EXPLOSION. THE DECK SHAKES]
SISKO: Don't bet on it.
Then the
Defiant shows up to pound on the Breen ship, some conduit behind the wall in the room blows up which knocks out the guards and distracts the Breen leader, Sisko wrestles the gun away from him, manages to beat the hell out of his former captor, and then escapes the Breen ship before it explodes.
Defiant picks him up, they're all happy to find him alive, and he talks about his discovery with his staff but they all agree to keep the secret in order not to de-stabilize the Alliance... because Gowron is a dick and would go off half-cocked, I guess, or maybe because there's a story reason why Sisko exposing what he learned about the Breen would also expose his Romulan trickery.
Of course, they never took this approach in the TV series, and from what I understand from hearing about the Typhon Pact books this possibility is shot out of the water (but please don't spoil me on it, I'm just finishing the fourth Titan book now, and I'm reading Destiny next, so I'll probably get to Typhon Pact in a couple weeks).