It would be a northerner, or possible a westerner, to balance the ticket, and likely someone relatively moderate/conservative. FDR would be ideal if he were an option. Newton B. Baker, Woodrow Wilson's pacifist Secretary of War, might be an option, since he was mentioned as a compromise candidate for the presidential nomination. Apparently Williams Gibbs McAdoo wanted to be FDR's running mate, so he could just as easily serve in the role for Long.