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.
The Klingon Empire is one of their oldest neighbors, and in two centuries they seem no closer to seeing any appeal to the Federation. The best that's been achieved in that time seems to be a strained tolerance, which crumbles under even the slightest outside pressure. With Romulans, they don't seem to have achieved even that much.