I'm deffinately looking forward to trying this. Actually it sounds like you could have a really great game concept there, and perhaps include some other spells or scenarios. For instance, bating magical creatures with specific spells, say drawing a flame shape for a fireball, a strike for a lightning bolt, a jaggid diamond for an ice blast etc.
I also like the magical letters idea, though describing the shapes of capital letters could be interesting.
It just sounds like you've got such a unique concept, it could be made into several crazy ideas and situations.
you could also increase difficulty by having a colour or element change, so that one shape required drawing lines of a different element.
So for instance, lefft mouse button scrolled forward betwene ice, fire earth and lightning elements, and drawing the mouse produced a line of that element.
shapes with multiple lines of different element types would be especially hard to do sinse you'd need to remember to switch your drawing element in the middle.
Btw, as an interesting fact, mages in the steampunk magic podcast novel The gearheart found here actually do! draw shapes in the air to cast spells.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)