I like the idea of Humans bein in the middle. If it were completely up to me (which it ain't), I'd have a generic magic unit equal in strength to the gunpowder units (or a lil below with a bonus gainst them), then a generic gunpowder unit, and a pure archer unit for civs like the WEs. Then I'd have a UU for the Dwarves (Gunpowder UU), WEs (Archer UU), and Thay (Magic UU). I'd also have a seige unit that is gunpowder, like a cannon, and an upgraded version of the trebuchet equal in strength, (insert name here). The Dwarves should have a UU cannon unit to show their extensive use of gunpowder.
Those are my thoughts. I'm jus addin onto wat dead said, kinda formin somethin based off of his thoughts. But everyone tell me wat ya think.