i love it!
1) for new terrains, there are Hobbes' mods, Terrain Pack (discontinued?) and UFO Redux, you cold look into them to see what can be done.
1b) what you should worry about is the limit of total MCDs for the MCD library of a map (in MapView), which is limited to around 250 (actually it's a bunch more, but yeah).
after those 250~ish MCDs, there can only be "destroyed versions" of the tiles, and you can't place them on map. see attachment (courtesy of XOps).
sort of confusing i know, my advice is simply to not have more than 250 total MCDs.
sprites aren't really limited, since a single MCD can use up to 8 sprites (if animated), so theoretically you could have 250 * 8 sprites.
the best thing is to just pick an already-finished terrain, and work on it. Hobbes' mods have some neat urban terrain that might be useful.
2) i think that regional cities should be doable. not 100% sure though.