If the legends are indeed Dragon-typed, then I'd like to see Zekrom be Dragon/Fighting. At first, the obvious type is Dark, but besides its coloration, what makes it seem "Dark"? Of course, the next obvious typing is Steel, but Dragon/Steel has already been taken by Dialga, and I doubt they'd do two consecutive cover legends with the same typing. Dragon/Fighting is a unique typing, and has completely unresisted type coverage, with the only possible resistor being a Steel/Ghost. For Reshiram, I'm not sure.
However, if it turns out they're not actually Dragon type, then I'd love it if Zek was Dark/Fighting (unique typing, near perfect coverage), and Reshi Ghost/Normal (unique typing, good defensive typing).
Just my two cents.