There really aren't a whole lot of useful pre-1400 practical alloys that aren't in the game already, or that aren't variations on a theme (i.e., varying the tin content in bronze).
About the only historical and practical alloy I can think of is arsenical bronze, made from a combination of copper and arsenic. Realgar and orpiment are arsenic ores. Too bad poisons aren't implemented.
A lot of cultures' metalsmith gods, like Hephestus/Vulcan, are deformed, lame, and/or mad, since many early copper- and bronzesmiths would start suffering the effects of arsenic poisoning after many years. Even if they weren't intentionally making arsenical bronze, many copper ores have arsenic in them already.
I guess you could also make certain rocks be ores of mercury, though it's not yet possible to do reactions which require a container like a vial, as mercury should. Don't know what uses mercury amalgams would have in DF. Lots of crazy people throughout history were dumb enough to drink it, just because shiny has to = good.
Hm, nickel is in the game. You could add meteoric iron, which is a combination of iron and nickel. IMO, it should make armor that's nearly as good as steel, but weapons that aren't that much better than regular iron.