Something that occurs to me: there's a lot of human exceptionalism floating around. So often, if there are a number of alien civilizations in a story, humans are the most powerful (at least out of those still around and showing their faces). The usual explanation is that everyone else just advances so slowly (even Orion's Arm uses this justification). But I'm not convinced. Why us? Why are we so special?
I've decided to challenge this in the timeline I talk about a lot, but never write. So here's sort of a general worldbuilding concept.
Don't get me wrong, I'm not completely disavowing human exceptionalism. Humanity (as a culture, not a single biological species obviously) gets their few thousand years as masters of the known galaxy. But even at their height, there are many parts of the galaxy where human influence cannot spread, where other powerful influences block their path or limit it. And nothing lasts forever.
By about fifteen thousand years from today, human metacivilization is declining. The ancient dream of humans spreading across the galaxy was once prevalent, but no longer. Eventually, a harsh reality check came in the form of other alien metacivilizations with similar goals, or at least a strong determination to hold their own borders. Human expansion dwindled, then halted, then began to reverse.
Meanwhile, humanity was buckling under its own weight. It had spread across so many thousands of light years, and diversified to a mind-bogglingly vast degree from the original biological Homo sapiens. Cultural mixing with a variety of aliens enhanced this effect. Many human civilizations no longer recognized their common heritage, regarding the notion that all humanity came from a single planet called Earth as absurd and even blasphemous in some circles. Human metacivilization was about as large as a metacivilization descended from human-like minds could ever be; it was on the verge of breaking. Inner civilizations were seen as decadent and complacent; outer civilizations as paranoid and mistrustful.
Why the decline?
To put it simply, humanity was growing old. We were millennia past our prime. Even by the time of Pax Humana, the frenetic rush to conquer new star systems was slowing down to a slow, steady march. Eventually, it all but stopped. Other civilizations were emerging across the galaxy. They were younger, more aggressive, and more expansionist than humanity. Many of them had shot from first agriculture to the first singularity much faster than humanity--in half the time or less. Admittedly, this often came at the price of an extremely bloody rise to Type I status, with a society roiling in constant turmoil.
As humans and human-derived intelligences fell back, the youngest generation of starfaring civilizations pushed forward. Many systems that were once ruled by humanity changed hands. Such exchanges were sometimes violent, but not always or even usually. Economic and cultural warfare were much more common. There was no concerted effort by these new metacivilizations to destroy or conquer humanity. Just a steady, inexorable march outward in search of resources. Humanity was just a slower-moving, less adaptable metacivilization, one of many scattered across the Milky Way, most of them meeting a similar fate. We were no longer the colonists, but the colonized.
System after system slipped out of humanity's grasp, even the ancient Sol System--already sparsely populated backwater by this point. The human-derived aigods bore the brunt of the cultural shift, either merging with younger alien gods or being culturally altered by them. Another five to seven thousand years and human metaculture ceased to be an important force in the galaxy, confined to a few scattered, minor human civilizations of no more than a few thousand star systems each.
Even though the overarching human metaculture was all but gone, pieces still lived on. Cultural mixing goes both ways, and even as the new metacivilizations slowly erased human metaculture, they adopted some of it. And even in areas where this metaculture no longer existed, there were still pockets of Homo sapiens and the myriad derived subspecies, minor ais, and human-allied aliens still living and even thriving. But they were no longer human, just as the Romans were no longer Romans after the fall of Rome.
There was no final collapse, no date when the human supercivilization was dead. Just human aigods and major human civilizations winking out one by one, or becoming part of something else. We had a good run of more than twenty thousand years as a spacefaring civilization, nearly thirty as a civilization. But all good things must come to an end.
As Shakespeare divided the life of a man into seven ages, so too can the life of mankind be divided into seven ages. Once upon a time, primitive tribes of humans had wandered their homeworld, hunting and gathering and relying on mysticism to explain the world around them. Then humanity passed out of infancy and into childhood. They still sought to explain the world around them, but with logic and experimentation, not myth and legend. And they explored their world, and formed nations, and developed an industrial economy. They began to look to the stars in wonder.
And so they entered their adolescence. They took their first teetering steps beyond their planetary cradle, then went further, colonizing other planets, and eventually other star systems. Their economy became post-industrial and information-based and they harnessed all their planet's energy, and more. But adolescence is always a chaotic time. It was the only time when humanity had weapons that could ruin a planet, but no way to survive a full-scale planetary or interplanetary war. Meanwhile, another chaotic shift occurred as the first posthumans and high ais took power from the baselines, and eventually lost it to the first aigods. But humanity was one of those civilizations that survived its troubled adolescence.
And so they entered adulthood. They invented, then perfected, methods of traveling faster than light. With this, they began expanding outward in a frenzy, wildly discovering and settling myriad new worlds and building truly massive works across the stars. Countless aigods vied for the bounty of the stars. They utilized the entire power output of their home star, and eventually other stars. They found brothers among the stars, in the form of allied alien metacivilizations, and they found children, in the form of extinct aliens to be resurrected and primitive aliens to be uplifted.
Eventually, they entered middle age. Their wild expansion slowed to a steady march and human metacivilization began to concentrate more on developing what they had. The aigods continued to grow grander and more powerful, presiding over an ever-diversifying base of lower intelligences. These included many "child" races who eventually grew up in their own right. They still had their squabbles, but they relied more on efficient techniques of subversion and cultural warfare than direct combat.
After a long time, humanity became old. They no longer had the strength, adaptability, and resilience needed to compete on the galactic stage. Pax Humana was over. Territory they had once held was theirs no longer. Cultural dominance they had once held was theirs no longer. Younger races, cleverer and more ruthless, were emerging as a new generation of metacivilizations grew up. And finally, humanity entered its seventh and final stage, slowly losing the last vestiges of cultural significance and physical territory, falling back to a few isolated bubbles scattered across the Milky Way.
It was a bittersweet time to live, those last few thousand years after the sun set on humanity. There was a sense of sadness that it had to end at last, but there was also pride that we had lasted so long. They had once been an immature child of a civilization, confined to some distant unknown planet lost to the mists of time--Old Earth was relegated to semi-mythical status by this point. But across the vast gulfs of time, they had grown up, and into an old and wise metacivilization. And for the people and civilizations who were willing to become something other than human, it was the beginning of the next great adventure...for them, it was not dusk, but mid-morning, and a long day awaited.
In the end, humanity did not die a violent, premature death as many once feared, but a peaceful, natural death of old age, after nearly thirty millennia of civilization.