I don't even know if it would be possible, given the way freeways are generated, but I'd like to see something done about their terrain following properties. Right now, rather than follow the terrain, X-Plain generates bridges and elevated sections to keep the freeways level. While this might work for out of the way backwoods areas, it looks pretty odd in places that are familiar. I live in the Seattle area and the entire I-5 corridor down to Tacoma is about 95% elevated in the sim.
@JonRD463: Yes this is a known bug. You can read more about this here (see "Crazy, Deformed Roads"):