The Breen (and my problems with them) (SPOILERS)

[Sci]

^^There are also:
Kshatriyan/Ksahtryan Regime
M'dok Empire
Tarn Empire

Quite possibly also:
Goeg Domain
Taurhai Unity
/Sha'Kurian Ducal Territories/

and who knows how many minor/medium states were not mentioned but do exist and could be vital....

I think at this point, all the major and medium powers have already either taken sides or declared neutrality. These other states all appear to be small -- controlling perhaps a handful of star systems, not large enough to affect the interstellar scene on the basis of their own strength. The power that smaller states will have is either going to be strategic (e.g., the Venette Convention being located adjacent to Federation, Ferengi, and Cardassian territory), or to be had in large numbers of them acting in concert -- either by aligning with either side, or by joining together as a sort of interstellar version of the Non-Aligned Movement (or a Star Trek version of Babylon 5's League of Non-Aligned Worlds.)

 

[Tim Walker]

Perhaps a more prickly version of the League of Non-aligned Worlds? A grouping that has enough collective strength to be a real player in astro-politics?

 

[toughlittleship]

The Jibetian Confederacy from DS9/The Long Night seemed powerful, but was applying for Federation membership at the time of novel, circa DS9 Season 3.

 

[rfmcdpei]

Perhaps a more prickly version of the League of Non-aligned Worlds? A grouping that has enough collective strength to be a real player in astro-politics?

From my understanding of Babylon 5, that would require significantly greater coherence than any of the powers listed here could muster. Most of the League members had either been colonized by the Centauri or had held them off during the Republic's expansionist phase, while more recently they had faced an existential threat from the Dilgar. The shared history of the League made it easier for it to work as a coherent body.

What do these powers have in common? Scattered literally all over the map, with histories of conflict and cooperation with very different powers, there is nothing that would seem to unite them. Some have histories of conflict with the Federation, while others have histories of conflict with other powers including the Romulans, the Breen, and the Klingons. I can imagine some of the less isolationist of these powers gravitating towards one bloc or another, maybe.

 

[Enterprise1701]

Tarn Empire

I don't count this one because they only appeared in The Forgotten War, which is completely incompatible with subsequent canon and novels.

Goeg Domain

William Leisner himself has indicated that the Domain probably no longer exists in the late 24th century.

Sha'Kurian Ducal Territories

I also don't count this one because Klingon Academy is incompatible with the novelverse.