What's Q like here? The Dominion, Borg, Romulans, Tzenkethi, Iconians, Organians?
I would reckon that the Dominion, Romulans etc would be like the Federation are now and would be at war with an evil form of the Federation. A total role reversal.
Here's my take on the Mirror Romulans: The TOS Romulans were originally supposed to be the “good” bad guys. Although warlike and ruthless, they were somewhat admirable for their bravery, honor, integrity, and stoicism. They seemed a pretty grim and humorless bunch, and they had that classical Spartan/Roman thing going (the Rommies in “Balance of Terror” even had Latin-sounding names). So I figured the MU Romulans are turned around 180 degrees -- they're like silly, effete Romans or Greeks from a Carry On movie. They're cowardly, conniving, and comically inept. Their plasma weapon is always blowing up in their faces, and their cloaking shield frequently malfunctions so they think they're invisible when they're not. They're the clowns of the galaxy, bungling their way through space.
As to the Klingons, originally they were vaguely modeled after the Mongol Empire, and they were meant to be the
real nasties. So I see the Mirror Klingons as something like Shaolin monks, dedicated to peace and nonviolence, spouting ancient Asian-sounding wisdom and philosophy. They're all experts in martial arts and know 100 ways to disable an opponent without causing death or permanent injury. And they're a race of colossal bores.
Of course, the roles of the Klingons and Romulans essentially switched places with the coming of TNG and the Federation-Klingon alliance, so what the two races are like in the post-TNG MU is up for grabs.
. . . This is the way I always saw it too. Not just a different set of circumstances, but just everyone is basically more evil, even the Mirror Mother Teresa would be evil!
Mirror Mother Teresa kicking cripples, spitting on orphans, and stealing from blind beggars in the streets of Calcutta? Now,
that I'd like to see.
(Ooooh, I'm such a sicko!)
. . . In essence, it's a very interesting concept that can work very well if the writers don't get too carried away with black leather, girl-on-girl action and OTT moments to the point of not taking it seriously at all anymore... which, unfortunately, they did on DS9 after the first MU episode - which is why every new MU episode sucked harder.
But one thing's for sure -- you can never have too many female bellybuttons!