Group Selection will be a huge timesaver. Import/export with JSON and/or XML is all I need. Another graphical format option to consider would be SVG.
Idea: It would be helpful to somehow mark terrain tile pieces that are either completely hidden (entire top surface and all sides) or partially hidden (entire top surface, but not all sides). This way, someone could use different tile pieces for completely/partially hidden tiles, and still create a map that looks the same, but with a slightly different assortment of tiles.