Out of sheer boredom, and curiosity as to just where the KDF might draw additional races in the future, I did a few hours of research to find some potential candidates. I'm sure this has been done before, however maybe not to this scale. Some of these selections may be fanciful, or far fetches, but hey, when you have aspergers the sky is the limit, right?
I've included some of my logic for suggesting these races, and It'd be interesting to see some peoples thoughts on individual races as they might fit in within the KDF, and possible traits for those that seem likely to you.
I think we all already know some are
extremely unlikely, so just gloss over the ones that area laughable whimsy, if you'd be so kind.
http://memory-alpha.org/wiki/Antedean
Antedeans: A race known for unsophisticated thought patterns, and a distaste for space travel, as well as known for their assassins. Considered a handsome race by the Klingons. (Or at least Worf.) If their need to hibernate through space travel could be circumvented, these gorgeous fish men would be a welcome addition to the empire.
http://memory-alpha.org/wiki/Antican
Anticans: A race of carnivorous, furry predators who like live prey, including that of sentient species, like their rivals, the Selay. They'd applied for federation status at one point, but it would be interesting if mutual conquest of them and their Selay rivals brought both into the KDF as rival races within the empire. Eager to make themselves more useful than their rivals.
http://memory-alpha.org/wiki/Chalnoth
Chalnoth: An anarchal society of hideous, lion like humanoids who are so vehemently against government they have none. Only the strong survive. Beaten into submission, they'd make a nice addition to the empire.
http://memory-alpha.org/wiki/Ellora
Ellora: Former S'ona slaves might seek protection from a more powerful force, like the KDF. Or, if the S'ona themselves alligned with the KDF, they might come along for the ride.
http://memory-alpha.org/wiki/Flaxian
Flaxians: A visually distinctive race, known as incredible assassins creating incredibly complex kits if 'perfumes' which can create varieties of poison when mixed in various quantities. Also known to be excellent with explosives. As soldiers of fortune, they'd probably be attracted to wealthy clients. Such as... Klingons.
http://memory-alpha.org/wiki/Kzinti
Kzinti: Larry Nivens superb warrior cat race from the Known Space series, also made its way in Star Trke lore in the animated series. A giant, angry race of cat people, possibly related to the Caitians. In the same sort of way that my Tabby Cat is related to a smilodon. Technologically inferior to other races, they might easily be folded into the empire if given the chance for renewed conquest.
http://memory-alpha.org/wiki/Miradorn
Miradorn: A species of angry, argumentative smugglers who hold a serious grudge. I see promise here.
http://memory-alpha.org/wiki/Na%27kuhl
Na Khul: A race who eventually enter the temporal cold war, and have an extreme vehemence for the federation. Not a bad idea to aid the Klingons in their war against the federation, perhaps.
http://memory-alpha.org/wiki/Rasiinian
Rasiinian: A race which won a 'design a species' contest way back when, very mysterious appearing, might add some interesting flavor to the KDF
http://memory-alpha.org/wiki/Satarran
Satarran: A primitive race with ambitions of conquering their enemies, the Lysians. Perhaps even eager enough to make a deal with a far more powerful empire in order to achieve this goal.
http://memory-alpha.org/wiki/Selay
Selay: Scary carniverous snake men , rivals of the Antica, who have the gall to eat Selay as a food source. Bitter rivals who, if both inducted int the KDF, would make an interesting pairing, what with an intense rivalry that exists between them. Theoretically applied for federation membership, but did you ever see snake men in starfleet?
http://memory-alpha.org/wiki/Skorr
Skorr: A race of warrior bird people who once had vast armires powered by advanced technology, however a recent philosopher turned them away from their warlike path towards a more peaceful path, however there have been signs of throwbacks. If conquered by the KDF, they might make an interesting addition to the empire.
http://memory-alpha.org/wiki/Son%27a
S'ona: While many of them have regained their youth, I imagine many of them still feel the lust for empire that they and/or their parents felt when they left the B'aku. After their losses during the events of Insurrection and the Dominion War, they might take their technical expertise and cunning, possibly even their slaves, with them somewhere where they can accomplish more.
http://memory-alpha.org/wiki/Tarlac
Tarlac: like the Ellora, with the S'ona destabilized as power in the alpha quadrant, their former slaves might be looking for some new coat tails to ride. Or, if the S'ona themselves aligned with the KDF, they might come along for the ride.