Really, I think the answer is "anything." Though construct familiars certainly make sense, especially for the mean flesh-hating warforged. And some folks on the WotC boards toyed with the idea of low-level living spells as familiars, which would fit any warforged from the Mournland.
Just don't think your warforged has to have a construct companion, whether he's a wizard or a druid or a ranger.