Another idea might be to allow the DFA to turn while in the air, but to lessen the power of the strike as you turn away from directly ahead. For instance, if you launched a DFA, and your opponent dodges to your left 45-degrees, and you turn to follow him, then you'll lose some of the inertia of the swing, and thus some of the CSC bonus and damage associated with the DFA. It'd have to be a pretty significant amount. A 90-degree turn would have to render the move nearly useless.
Great idea. Coupled with knockback/knockdown if it connects, it might bring the moves cost-benefit back into whack. It could be a very nice compromise between the needs of duelists and ffa'ers.