I don't have any good idea for a big, unified post today so allow me to give you a few little snips of ideas.
1: A Tidal Rush: A land where a Mighty river flows through the jungle. Once every three years the sun and moons align and create a "negative tide". A tidal surge flows some 200 miles upriver, wrecking the boats and homes of the unwary with unearthly riptides. Gnomes surf these waves for fun.
2: Mermen: Are mermen just Men who have naturally selected to dive comfortably to 300 feet and stay underwater for six or more minutes? They would be small, brown and sun-baked, and nearly naked despite being quite modern and sophisticated.
Maybe instead they are elves
3: Alternate Orcish: Orcs are like the Neanderthal. Just as smart, 2HD but inferior weaponry - they were terrible shots and couldn't make good bows. Men with superior technology subsumed and interbred with these original men, and so half-orcs now live. What advantage did Men have over the Orcish? Community. Because we had symbolic artifacts like religious figurines and bone flutes, our culture made it possible to work together and share labor in a new and efficient way.
1- religion and culture
2- division of labor
3- more extensive political groupings
4- superior technology