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What if Damar had not killed Ziyal?
Darn...I wanted this one! I'd actually written a fanfic with that very premise.
What if Garak remained within the Obsidian Order and Dukat was abandoned on DS9 when the Cardassians withdrew?
Dukat is immediately sent to a war crimes tribunal on Bajor, where he is executed according to Bajoran law over the thunderous protests of the Cardassian people. Over the course of a few years, his death--and the removal of his influence--emboldens resistance leaders on Cardassia like Tekeny Ghemor, Natima Lang, and Aamin Marritza (who had been considering a drastic plan to force Cardassia to face up to its crimes, but felt that it was no longer necessary after Dukat's death). When the Dominion is discovered on the other side of the wormhole, the Obsidian Order and Tal Shiar still try to take on the homeworld in the Omarion Nebula, and Garak dies leading the battle as Tain's "general." In this power vacuum, the Ghemor-Lang-Marritza coalition takes power and initiates reforms. But this time, when the Klingons try to invade, the Federation offers much more than a halfhearted defense: they defend the reforming Cardassia both with intelligence showing how long this emboldened resistance movement has been acting, and with military force. Confronted with this evidence, the Klingons are forced to back down lest they lose face.
Instead of facing a divided quadrant where they can gain a beachhead, the Dominion instead faces a united quadrant. They attempt to continue sowing paranoia through acts of sabotage, but ultimately the reality is that they are trying to fight a war against a quadrant to which they have only one access point, which is mined over Bajoran objections. Once the minefield is deactivated, the Federation--instead of having to worry about a Cardassian front--can instead think about taking the fight to the enemy in the Gamma Quadrant, allowing them, after a bitter, grueling war, to deal out a more decisive defeat.
P.S.: Dukat's daughter is eventually found on Dozaria by Major Kira, and formally adopted by Kira when it becomes clear that because of her father, no one else is willing to care for her.
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What if Odo were executed for treason by Major Kira, for conspiring (and co-mingling) with the Female Founder?