One of the issues with supporting multifaction content in STO is that, no matter how Cryptic slices it, fewer players will play non-Federation than Federation.
It would not surprise me at all if the total number of Cardassians, Romulans, Dominion, and Klingons combined is approximately equal to or less than the number of Federation players. (And if any growth incurred by adding factions doesn't also increase the Federation totals to keep the total number of non-Federation, in the most ideal scenario, to less than half of the population.) It would not surprise me because I've played and been an administrator on fan run Trek games going back to the MUD days and that is how those populations distributed in many games. Some games avoided the pitfall by opting for a single faction and social roleplay focus while more PvP centered games almost always had to institute quotas and waiting lists to balance out the factions... or adopted a model where NO players played Federation (ie. Federation was restricted to NPCs and administrator control only), resulting in fairly equitable distribution among the remaining factions.
However, there is an additional solution in a game like STO and it boils down to an analysis of the official map that Cryptic uses as inspiration in sector design:
What you will notice is that the empires that were portrayed as equally matched on the shows do NOT occupy equal space on the map. This is partially because the Klingon Empire stretches a bit south of the map's limits. (However, the Federation does as well.)
You could chalk that up to popularity. You could chalk that up to number of episodes the characters were featured in. (There is some relevance to this idea in that the maps were constructed by trying to place adjacent episodes in close proximity to one another while also cross referencing maps that were seen on screen.)
By that standard alone, the Cardassians are far too small.
However, there is a simple solution and it both fits canon tendencies and applies to content development for STO.
Federation space is not wholly definable as the space the Federation controls. Rather, it's basically all explored space that the Federation encountered minimal resistance in. The Federation scouts quite a bit while putting very little emphasis on HOLDING. They zip from planet to planet but expend no resources conquering and minimal resources recruiting.
The Klingons follow more of a pillaging pattern but slow down on their expansion more because they are more concerned with absolute control of the territory they seize, in particular agriculture, livestock, and weapon making resources. Still, they do some zipping around but at a slower, more confined rate. Even then, however, the Klingons don't fully occupy or demand absolute fealty from every world in their space and will ultimately just blow up someone if pushed.
The Romulans are more interested in fealty, in the undying loyalty of their subjects. The Romulans, like the Romans, are interested in trade routes and tributes from the worlds they occupy... and they occupy a few less because of Hobus.
The Cardassians are all about control. A given officer will spend his career in an entire sector or even decades on a single planet. Democratic reforms have likely shrunk their sphere of active control even further while focusing a redoubled effort on the planets they control.
In a nutshell, what does this mean for content?
The Federation visits a new planet every week, which has required lots of maps.
The other factions, in order of popularity, are more likely to spend more time in fewer places. Meaning less maps.
The Federation patrol. The Klingons, Romulans, and especially Cardassians spent far more effort holding than patrolling, focusing their equal strength to the Federation over a smaller area.
Take the existing Klingon maps, for the most part, and you can create new missions for them.
Take "Alpha." You could have five episodes built around that map, with an officer spending an extended stay there, hopping between ground and space. The Ketha Lowlands and orbit of Qo'noS could support more missions without additional asset development.
This is somewhat more true for Romulans, where ten level blocks could largely consist of one or two ground and space maps each and a focus on intrigue over a very narrow span of terrain per front.
By the time you get to Cardassians, practically an entire tour of duty could be mined out of maybe three to six REALLY well done maps consisting of a fairly fleshed out Nor, a city region with an outlying cave or two, and the space around it, with a career based on prolonged, focused occupation of a single system and prolonged relationship with a set of NPCs, really hammering home the DS9 showrunner ethos pushed by Ron Moore, which said, "We don't go places. We stay put and see what happens in one place, dealing with consequences."