I alluded to this in another post I wrote, but you're generally right--there's no such thing as an unbalanceable concept.
However, an important consideration to take when finding that happy medium between OP and UP is whether that ability still feels good to use in the end. Lee Sin is a great example for a champion that's actually very easy to nerf, as he's quite mechanically bloated...
Q: damage, dash, execute, reveal
W: shield, dash, energy return, life steal, spell vamp
E: damage, reveal, attack speed slow, move speed slow
R: damage, targeted interrupt, AoE interrupt
It's incredibly difficult to nerf individual numbers, as that runs the risk of watering it down so much he's no longer fun to use. However because his abilities have so much tacked on, it's a simple matter to remove one of the more minor aspects without destroying his core gameplay (for example, the energy return on his W that most people, including me two days ago, don't even know about).
Someone like Blitzcrank is really much harder to nerf because his strength is also his defining characteristic. It's up to live team to determine if Blitzcrank is a problem though.