Think of a civilization ruled by cloud giants. What would humans do in it? Humans are a lot smaller than cloud giants: maybe they'd be used for the delicate tasks - tailoring, lock-making, clock-repair etc. - that giant fingers are ill-equipped for. How about a civilization ruled by dwarves? Humans are more creative and artistic: maybe they'd be the entertainers, dramatists and painters. How about a civilization ruled by elves? Humans might be their warrior class, doing all the fighting for their risk-averse long-lived rulers (you could easily imagine elven city states fighting vast wars all entirely fought-out by human underlings). Maybe in a civilization ruled by derro or dark elves there would be space for human beings as tenders to the sick; no self-respecting derro is going to look after a fallen comrade, but humans might.
In such a world, human PCs might be looked upon as vaguely exotic, but not very special, outsiders suited to certain roles but firmly on the periphery of society. How they navigate that world might end up being just as interesting if not more so than the dungeon-delving or whatever else they got up to.