I know that this has been discussed a hundred times, but I still feel it just as strongly...where are the Cardassians? They're one of the most prominent races in Star Trek, we have playable Cardassians, Intel Ships, there's still the old Cardassian Arc...but what about the rest?
We know at the end of DS9 that Cardassia Prime was decimated, millions or billions of Cardassians were dead, even though they changed sides at the eleventh hour, I'm sure no one in the Alpha Quadrant trusted them and their entire race was in shambles. Honestly...they sound like more of a direction, than the destruction of Romulus!
The Obsidian Order was destroyed...but maybe it rebuilt in secret. The Cardassians were made pariahs...but maybe they pulled themselves up by their own boot strings. Not to mention that instead of Mirror Leeta and the Pah-wraiths, why couldn't Dukat be brought back?
I'd love to visit Cardassia Prime and see it as a wasteland, or being rebuilt. The wasteland would really be ironic, after how the Cardassians left Bajor.
Cardassian technology was pretty unparalleled. Until they shut the power off, their defence platforms were obliterating the attacking ships and Dreadnought? Ooh, but Dreadnought was UNSTOPPABLE and if only they'd built more. Maybe though...one appears. Then another and another and another, by Cardassians, surviving Obsidian Order or a rogue element who want their power back.
And instead of cloned Klingons...how about cloning Seska? I always assumed she was OO herself and there are all sorts of weird and wacky ways to do that.
In the novels what Dukat was doing to the real Iliana was CHILLING, but there's just so much scope!
I also bet that Andrew Robinson would return as Garak...and maybe even write the arc himself.
It could go in any direction, but a Cardassian season would be lush.