I think they said there would only be one new job launching with 4.0. Or at least that they never intend to introduce 3 new jobs at the same time. They also said that 3.0 would either have a theme of ocean or sky, and they chose sky. So 4.0 will likely involve water and coastal locations.

Whatever the case may be, we'll probably get a hint when 3.1 launches, assuming they continue the tradition of poetry hinting toward the future expansion.

Personally, I doubt we will be seeing Doma or the Garlean Empire, until we at least see a proper map of the world somewhere. Eorzea is the western most continent, but we've never even seen a proper outline of the central or eastern continents.