There are champions that can 100-0 most of the champions in the game, but if you're talking about, no matter how far behind you are, if you d/c for the first 20 minutes of the game and hit lvl six when everyone else is max level full build, and you still have the ability to one shot anyone, I'd say it'd be pretty hard to balance that champion, but it could be possible.
This may be a loophole in the champion design, but consider a champion the champion I outlined above. They would never need to go past lvl 6, presumably, since once they get all their skills, they can one shot anyone. That means, they neither scale with gold nor experience, getting kills will do nothing for you without the ability to take objectives. What could potentially balance this champion are incredibly long cooldowns, so that it can only be used once per teamfight. OR another way is to make sure this champion dies with whoever they kill. It's rather silly, but so is this champion. Doing this ensures the game is almost perpetually 4v4. Most likely too, this champion would be building tank, since they have true damage and don't need any damage stats. This champion too should probably have incredible risk to their kit, maybe they take increased damage while executing their skill, or their skills take a large portion of their health to execute (for example, if each one of their abilities took 26% of their health and they are unable to be healed and all shields removed when they initiate their combo (to adjust for health regen, etc.). Zilean ult and GA would have to also not work for this champion. The difficulty of landing this combination would have to be immense as well.
I want to say a champion of this nature could be balanced, but the cons of playing them probably would be so immense that I don't know why anyone would play them.
I'd say difficulty is pretty relevant for champion balance. Balance changes to LeBlanc have been to remove her silence, allowing for more counter play to her kit. Ziggs and Nidalee have had the diameter/width of their Q reduced, making their power a little harder to access. Many different ways to balance a champion, but difficulty of access definitely seems like one of them.