It's possible that there's some way of doing this that I've overlooked, but if not, could it be possible to rearrange the order of a defined terrain in a tileset? As-is the first-defined terrain is the first in the list, the next one is second, and so on and so forth. Especially with larger tilesets with many terrains in them, the ability to reorder them would be a big help rather than trying to plan the whole order out beforehand.
Similarly, tilesets added to a map are displayed in the order they're added, with no way of rearranging them. Being able to do so would also be nice, though obviously not the highest priority thing.