well see. that's the thing. We don't know what the armistice actually involved.
There's no way Odo would have gone to help his people (willingly) after linking with the Female Changeling Founder if it was to "rebuild the dominion to it's former glory." the J'em H'adar and the Vorta would probably do whatever Odo said as he is a god to them.
I'd argue the only real foothold that they could not control is the B'reen. why did they want to attack the federation? what were they promised (other than Earth). they don't answer to the Founders they could attack with that super duper destroy weapon at any time. - but again we don't know because we don't know about the details of the armistice. (and Cardassia is in no position to do anything, basically becoming like Bajor at the end of the war, having lost billions of people, all their resources and having to re-image themselves. I always liked that little irony the show did there).
an anthology about the aliens of star trek was something i've had an idea for ages. every 'season' is sort of like. what makes them tick. heck this way you could even have a Borg Origin story. (i know people would groan etc) - but i dunno. to me, the Borg is sort of like the White Walkers of Space. they had to have been created somehow (with the organic/synthetic pieces). or maybe they weren't made but literally had to adapt - having to graft machine to themselves. (sort of like the aliens in Voyager who had that super disgusting disease and they killed aliens to give themselves body parts yah?)
or like the other guy said - do a mirror universe.
pick moments in written history (or even talked about history - again. Pegasus) that you COULD go back to.
show us the cardassian wars.
hell. show us the DMZ skirmishes and (truly) what led to the maquis (the DS9 version was kinda lame, and only used for the gateway for Voyager).
star trek has a lot of great story ideas in books etc, that they easily could do whateve they wanted without focusing on Kirk et al, and Picard et al that would seriously intrigued any Trekkie but be different enough to capture a Sci-Fi geek + Casual Viewer as well. you just need strong writing (as you need really for anything on television).
The Breen were opportunists. They saw the dominion as a means to significantly expand their holdings. They were referenced early on as an aggressive state, so it wasn't seen as to out of the ordinary when they joined the dominion.
I don't think the dominion will change their attitudes. Odo probably knew this when he made his decision to return with the founders.
The dominion is still the dominant force in the gamma quadrant, and is easily on par with the federation in terms of power and the ability to project that power, at least in the gamma quadrant.
Because of their size and power, the dominion has no reason to back off on their current policies. The founders will not be changing their way of thinking over one instance where it became a necessity to do so. The changelings were a people who were hunted down and killed without remorse. The mindset they developed from these experiences will not change, atleast not over night.
I would love to see an episode centered solely around the vorta. While they're genetically programed to see any changeling as a God, they're a highly intelligent race and seeming quite the independent thinkers. That dynamic might make for a good story I think.