We eventually need a skill based off the Starcraft 2 Raven's Seeker Missile (details inside)

Basically, the missile works like this:

The Raven deploys a Seeker Missile that takes a few seconds to lock on to target enemy. If the enemy does not get out of range before the lock-on finishes, the missile flies at the target (it's now undodgeable) and deals damage in an area upon impact. If the target DOES get out of range before the lock-on finishes, the missile fizzles out and nothing happens.

This could lead to so many fun plays. Fleeing an enemy? Launch one of these Seeker Missiles at an enemy Champion during your retreat, forcing them to decide "do I keep chasing and get hit, or withdraw to avoid damage?". Obviously in League there would be more counterplay to it (i.e. become untargetable, shield it, etc.), but the very essence of the skill could lead to plenty of fun stuff.