Okay, I get what you're saying without going into the minutiae of comparable polities. If we're taking the US as an example, we have to remember that the US has a defense strategy of far exceeding anyone else in the world when it comes to military readiness, expenditures, capabilities. The UFP and Starfleet may be comparable, or believed itself comparable, and this led to the situation before facing threats from the Borg Collective and the Dominion (both multi-species, sort of, with capability on s level unmatched by any other). Because they're our heroes, they managed to defeat these threats.
But who are the "threats" the US faces today? Not Russia or China (publicly at least), but small groups. Al-Qaida. ISIS. Guerrilla fighters with no pretense of taking down the "rival government".
So the next threat to the Federation shouldn't be the Krenim Imperium or the Concordant of Galaxies. But someone much smaller that can fight our heroes existentially, if not completely.
But it would be a very inaccurate analogy to describe the UFP as having anywhere near the military superiority in the Milky Way Galaxy that the USA has on Earth. The USA has a far larger percentage of the land area, population, industry, wealth,etc. of the planet Earth than the UFP has in the gigantic Milky Way galaxy that it only rules a minute fraction of. There could be thousands or millions of other states with equal resources to the Federation out there, or possibly a single state with 359 times the military power of the F UFP or 1,284 times the military power of the UFP, or 45,915 times the military power of the Federation, or whatever.
The idea that the new rivals of the UFP would have to be raiders, or guerrillas, or terrorists, or some other small scale foes, is not correct. There is no reason to assume that the UFP must have run out of foes with equal or superior military might in this vast and vastly unexplored galaxy.
My favorite idea for a new rival would be the Dominion. I was very disappointed that the "The Search" identified the real Founders of the Dominion so quickly. So I like to think that the Changelings are not the real Foudners of the Dominion, merely a subordinate group that founded a province or state of the Dominion that was assigned the task of conquering the area by the Bajoran end of the wormhole and failed. So the real Founders of the Dominion may now assign the job to some other province or state.
You might also be interested in my answer h to this question: https://scifi.stackexchange.com/que...tch-to-another-planet-when-qonos-became-uninh
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