Pretty simple, really.
Instead of dumb-fire missiles without locking on, what if they were manually controlled? You can either lock-on with the middle mouse button, or you can shoot your missiles and have limited control of their direction once they are deployed (to the same extent as you would locking on). Or maybe it could be only implemented as an addition to the Rocketeer ability? You would drag your mouse in the direction you want the missiles to go. Manual detonation seems reasonable as well.
I think this would definitely make hellfire missiles be much more useful in a high-tier/competitive setting.