I'm a bit late in this discussion but I'll toss in my two cents. I think the biggest issue is finding enough diversity to warrant branching new human civs at all.
Take Feudal Japan and Medieval Europe for example. Both are differentiated by location and culture. Beyond that they use swords, spears, farm and keep animals, and build above-ground structures. They're also both noble-based societies. They're basically vanilla humans, except Japan would have katanas and buildings made of wood, bamboo, and paper. The same goes for the Middle East in that time period, which doesn't differ much. Vikings are a bit more unique, and the trade republics of the Mediterranean Sea are the most unique of the lot.
You could go through the trouble of statting out all those different weapon/armor/building material assets for a bunch of roughly identical medieval-period (ballparking here) civilizations, or you could go with something a bit more fantasy.
Why not a magic-based human race, and a normal medieval-style human race? We have Warlocks, so why not a race of human Archmages that summon golems and elementals, living in towers made of marble by their various constructs? Or perhaps the archmages just use the golems and elementals for defense, and count on their townsmen to do all the work? It could even employ a command structure that's not noble-based, such as some sort of ruling council.
In short, you could maintain a normal vanilla-style medieval human race, and then create a fantasy-based melding of the Warlocks and said vanilla humans.