Also, I wanted to bring this subject to the table a while back, but I dunno if this is also someone else's opinion. But anyway;
I think humans are pretty dull. Not just as how they are used in DF, but as a fantasy race in general. You know? You got dwarves, industrialistic small bearded creatures who live in moutains, dig, love the underground, gold and gems, value objects, praise hard work and get moods. You got elves, pointy eared bastards close to magic and nature, who embrace unity and harmony as a whole. Goblins, orcs and what not... And then you have humans. Who are... Just humans.
In DF, human villages tend to look exactly the same, kinda like viking outposts with no real consistency. A house can be assigned as a tavern, or another one as a hospital or a shop, it doesn't feel like an actual city, but more like a melsh of people who simulate the idea that the building itself is a shop/tavern/hospital. So maybe fleshing human villages would be great? I know this suggestion has been brought up many times before.
What could be interesting, however, is that each civilization has different styles of architecture and city design. Take Europe for example, they each country may be close to one another, but each have their own city design and building appearance. Human Cities could have districts, precincts, boroughs, blocks, etc. and even their own styles of buildings. Maybe something similar to yurts, or russian great halls, or Dojos or Haussman houses. Castles could be very medieval-ish or have a renaissance-esque attire depending on the civilization it has been built in!
As for humans themselves, I was thinking they could be given a little "something", like a fantasy feature that no other civ could have. For example; Civ A's people are more likely to have frivolous hair that grows to supernatural lenghts, while Civ B's people are all tied together by magic rocks they get at birth, leading them to form some kind of group-think. This could go from physical features, to magical features, to strange behaviors.