Firstly, thanks for that clarification. Not only are the Hydrans, Lyrans, and ISC not even options, but the ISC are pacifists and the Hydrans are sworn enemies of the empire. I COULD see the Lyrans... if not for the fact that they are SFU only.
Second, I support the idea that humans should become Klingon-playable. Think about it. We're humans. There are plenty of freelancers amoung us.
I would say Romulans, but it would make no sense having Romulans in the KDF.
We've already got them; Klingon players who have completed the Romulan Reputation gain a Bridge Officer. I'd agree with you, such an alliance seems highly unlikely, but then FED and KDF players get to have a Breen Boff, so lore isn't exactly a strong theme here.
Any race added needs to be unique to the Klingons. I thought it dumb to add joined Trills to KDF, as well as the sabertooth Caitians.
KDF really needs its own identity.
Well, I believe there was a Trill descent species that cater to the KDF, it maybe canon - also keep in mind that Dax was among the warriors - so why not several of the Trill race partake of the glorious empire.
Romulans - They can make a mix breed which is canon - There was a TNG episode that dealt with Romulan/Klingon Hybrids.
The Kzinti is still not out of the way - they can still push for the license to design them.
Ferasans are a JOKE and UTTER Tripe - I was sad when they introduce the Gothic Emo Thundercats - out of gazillion canon species from the show, they decided to go with a more cheaper, unimagnitive creation - They are NOT Canon and should not even be in the game.
All they are, are Purple colored Caitians with Punk hairdos and fangs.
This was brought up before, but I think that the KDF could use something like the Mazone. A single gendered race of world conquering, ruthless, cunning plant people that use everything from disguises, infiltration, all out force and even orbital bombardment, but are refined and elegant to a fault. That sort of thing would make strong allies of the KDF.