The Federation currently have 15 (well, actually 14 'cause Benzite Bridge Officers are non existent). The Klingon Defence Force has 7. Was thinking about this yesterday; more specifically about what other species could be added to current (and soon-to-be) factions. Wanted to share my thoughts here and see what others made of them (hopefully Cryptic will take a few notes too).
To start with the obvious favourite; the Federation. I might as well add Benzite here cause (since the birth of the game) Cryptic still hasn't got around to adding them yet. In addition to the Benzite, another species that I think would have been a welcome addition (and one that is a member of the UFP); the Deltans. They could act (with some small changes) as the Federation counterpart to the Orion.
Then the Klingon Empire. I can only think of two for them as they're not the easiest of a faction to get along with (from a galactic perspective). Suffice to say, two species that I think would work with the Klingons (and/or their current allies) are the Boslic (they're known to be mischievous) and I can see them in an easy alliance with the Orion. The second addition here would be the Talarian; they're a proud species and they're more warrior-like than the TNG episode seemed to give them credit for. Don't think they've ever been a fan of the Federation either.
Next up (and I'm feeling optimistic here, but go with me) the Romulan Faction (whenever that may be). Obviously Romulan and Reman are a given, and the Hirogen could be C-Store purchasable (thus they'd not be as common). On top of that, (and with the recent mention of their system) I'd throw the Suliban into this faction. I'd have no doubt there are remnants of the Kabal still in existence, and such would be perfect for their style of operations.
Another two ENT-era species I'd add to the Romulans are the Kovaalan and Mazarite. The Kovaalan are only seen by the way of their ships, though they're said to be highly territorial (the Romulans like their territory and its secrets kept hidden thus there is a similarity there), and the Mazarite (in the episode they were seen in) were being run by a corrupt government. Granted this would be some 300/400 years apart here, but depending upon how deep such a corruption went that could lead them into the 25th century and into an alliance with Romulan cults.
Calm down, the KDF will be getting the Kazon at some point I'm sure. lol
If there ever was such a thing as a useles species then it would be the Kazon.
They have the unique quality of being unworthy of assimilation by the Borg.
While the Borg basically assimilate everything, including Talaxians, they intentionally decided not to do so with the Kazon.