In real life, humans are the most adaptable creatures on the planet; even in pre-modern times we have colonized deserts, tundras, rain forests and hills. But in dwarf fortress, humans are wusses who have to have just exactly the right kind of low-land biome, and who cannot be bothered to explore beyond their comfortable little grassland/savana area [correct me if I am wrong]. So I have a few suggestions to address this, assuming it should be addressed.
1) Make it possible for humans to settle in biomes besides lowlands. Perhaps they could colonize along the edges of biomes such as forest or high hills. Yes, I can probably get this by messing with the raws or something, but the whole point of suggestions thread is to suggest things for vanilla DF, right?
2) Make humans more adventurous when the settle. RIght now, it seems that humans can't cross over inhospitable biomes to build a new settlement. They never seem to build roads across hills, mountains, deserts, wastelands, etc.
3) Make humans spread up and down rivers. Right now I have seen humans build settlements right on top of a river when there are no lowland tiles available for them, but I don't think they can spread their civilization along the river. Even though boats are not in the game, I think spreading along rivers could be modeled in worldgen.
As you might guess, I am interested in this because I like genning worlds with extreme characteristics (desert worlds, jungle worlds, etc.) If for instance humans could build a road over just a few inhospitable biome tiles and colonize an area nearby, they would be more robust during the history phase of worldgen, and you could generate more extreme worlds in worldgen without worrying about humans going extinct.
If I am mistaken about how humans work in worldgen, let me know.