So we all know spell crafting is a no-go. What I would propose as a compromise to this long desired feature is this:
SPELL APPEARANCE CRAFTING
In simplest terms, allow players to change the COLOR SCHEME of their abilities.
This would be incredibly simple for ZOS to implement (all you are doing is changing some numbers to alter the color of the particle effects).
Additionally, there is certainly room for CUSTOM MORPHS (like stamina Twilight who scratches with claws, different summons, etc.).
But the basic concept, allowing people to customize their spell appearance to match their character/mount appearance, would be huge, and cost next to nothing.