I actually only see the Federation continuing to absorb everything else around it. To be frank, I think this is one of the underlying themes of the whole franchise [that many people miss]: that all the 'bad guys' with their Authoritarian, militaristic states see the universal appeal of the Federation. At its core, its ideals appeal to all, it is simply a matter of time before the peoples of neighbouring states realize that.
However, I don't agree with this. For example, although, in the post-TNG time frame, DS9 depicted the Romulans cooperating with the Federation for the limited objective of defeating the Dominion, there was never any indication anywhere within canon continuity that the Romulan Empire would be absorbed by the Federation. Ditto even for the Klingon Empire. On the contrary, the only indications were that they would remain separate, even if allied, entities.
As per Sloan (Inter arma enim silent leges), referring to the romulans and the federation, not even allied. Back to being cold warriors after the dominion is defeated, instead.
About the lifespan of the federation:
Historically, empires/hegemones/super-powers fall either because they were conquered by an even stronger nation or because they overstretch.
With the federation, by the 24th century we see signs of it becoming overstretched - and, consequently, challenged by quite a few other powers. For example, see the federation exploring the galaxy and gaining powerful enemies as a result.
If the 24th century federation cannot change this trend, it will disappear as a major power within, at most, a century - assuming no authorial bias.
We saw, of course, apparently federation ships and temporal agents from as far into the future as the 31st century.
But - was their civilisation the moral successor of the 24th century federation? Or was it a warped image, created from the ashes of a fallen federation? A holy roman empire on the interstellar stage?
Braxton was arrested by his second in command with VERY little persuasion and no evidence. Not many individual rights exist by Braxton's time, it appears.