I prefer visual casting, and/or stances. but when I think about it, yeah sure, illusion makes sense to be invisible. I mean, an illusionist today bases his very tricks by not revealing what he does
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So how about different stuff for different schools?
Illusion - Just a twist of the arm?
Cunjuration - Maybe a dance?
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Destruction - Animation "pulling" the resource from the sky shaping it during the cast? Grab the fireball from the sky before throwing it.
Alteration - The good old glow?
Mysticism - Animation "pulling" the resource from his inner self?
Restoration - Animation "grabbing" resources from the natural elements around him?
Dunno, going a bit crazy here