I'm really hoping the particle system makes an appearance at last, I've had a plan on hold since last May for them.
500X500 Schematics is a big hell yes in my books (saves me having to cut ones that size into about 100 pieces to import), but I'm wondering if there could be a small addition of a seperate non-native-to-minecraft block just for the program: one with 6 differently textured sides and a seperate "terrain.png" equivalent to go with it. Its just something so if we were to import a dozen of them with different skins, that we could parent them together in such a way as to create a custom mob. Its just the closest vanilla block with multiple face skins is pumpkins with 4 unique faces, and it's tedious stitching together human models with their heads/arms retextured for the same effect and making all but one part invisible.