-Ranged hold button and release at desired power; allows anything from rapid barrage of smaller fireballs to a single giant flaming ball of doom using the same spell.
-On touch same as above but player is in physical contact of desired target when button is released.
-Flame thrower double click and hold on 2nd click (the cone range/damage could be increased or decreased by left/right trigger buttons on controllers or scrolling the mouse wheel forward or back.)
-Glyph, I imagine theses as preparations for battle not done while being attacked, hold button down while looking at the ground and the fire glyph begins to trace. At certain critical points in the glyph the spell button is pushed and held to desired amount. The points represent size of affected area, intensity, duration and perhaps an inner "trigger" circle.
Caution, charge a spell too much and it could do anything from simply fizzle out to blow up in your face. One spell could replace a dozen in the spell book. By reducing excess spell clutter and shifting focus to dynamic casting instead of spell selection, the magic system becomes much more immersion and enjoyable.