In a group interview with content creators in London, Game Director Ion Hazzikostas confirmed that Blizzard plans to allow Dracthyr to roll other classes besides Evoker in The War Within.
During the Interview, Ion confirmed that while that's the plan for the expansion, it won't be a launch feature, but he's hoping they'll be ready early on in the expansion.
A glimpse of this future happened on the BlizzCon Warbands preview, when Blizzard has a Dracthyr character wearing leather gear by accident.