I always thought there were other spell orders out there, only the empire has the 8 schools of magic. Somewhere maybe theres a country where they have Demonoloists or Elementalists as the norm. The Slaan/Old ones, seem to me when engineering the world, must have used some form of Elemental magic. I do know in Fantasy Battle, there were Elementals used in the battles. Maybe those types of spells became restricted or banned, who knows.
All it would take would be to look back at the old spell lists and class them as a form of hedge mage from the Border princes or any of the other areas of the world. Just cause a spell uses the red wind for example, doesnt mean a bright wizard knows that spell.
The warhammer world doesnt have realms of fire air etc does it? I know it mentions a realm of the dead, but is that just aethyr void itself? So anything not of the world would be from the magic realm?
I have only ever heard of one flying ship made by a mad trollslayer inventor. I doubt they last too long, what with so many type of flying creatures that could rip them open. There are spell of levation, all you need then is the wind to blow you were you wish to go. Just strikes me to take alot of magical power to do this. If there was a lull in the winds of magic, the ship would come crashing down.