So, now that we've established Toady does plan to have all sites playable one day, and that you can kind of give it a go under the current mechanics fairly easily, what extra is needed to make a proper human site playable?
Cities are pretty massive (wide) compared to fortresses and still need z levels going up and to a certain extent down (sewers and dungeons) so probably would need a 64 bit DF just to embark without crashing.
Perhaps the ability to zoom out to a wider site view to manage the surrounding fields and such that make up most of the space?
One thing which needs to be changed is buildings. Under Dwarf rules you have to micro-manage building walls and a roof around every outside workshop, humans should do that automatically but give the player the tools to design more exciting buildings manually.
What Fun should a human site have besides invasions? They won't be digging too deep, so not as many underground nasties except the occasional one coming up from the dungeon. Perhaps a detailed crime system with gangs and bandits playing a bigger part of gameplay. Do human lawgivers even design dungeons? Or are they built under your feet by unknown forces while you're busy growing crops...?
Human/elf/goblin fortress is a long way off so there's time to implement all the systems necessary before it becomes a thing but I don't think discussing the far future of the game is a bad thing. Toady needs to design systems to be fairly future proof so ideas now will only help development.