exactly what the title says, the ability to combine two different spells to create a new effect. say combine a bound dagger spell with a firebolt spell, and have the effect be a scatter shot of flaming daggers in all directions.
and of course, mixing two spells that dont belong together could backfire in your face or accidentally turn you into a bunny, wabbajack style.

That would be very awesome indeed, but there is very little chance of it being possible untill we have the Creation Kit