I definitely think the potential is there. But it takes more than just the champion to be able to 1v5. The 5 have to make lots of mistakes the 1 can react to. Everything from Item Builds, positioning, missed skill shots, and wasted cooldowns and summoners. Without making those mistakes, no 1 can take on the 5 alone. Not to mention all the things that have to happen to set up the 1 to have the items needed to burst and the 5 being denied the items they need to survive it.
Having a single champion who can simply 1v5 just because they're that strong wouldn't be in any way be a fair scenario. I've watched as an Ahri weaved her way around a team, alone, scoring a Penta all by herself. I watched an old school karma take on a 1v4 only to kill the 5th who came in to clean her up. I've watched an Orianna obliterate a poorly positioned team and would have killed all 5 had the ADC been anyone but ezreal who could shift over a wall, then flash over another to escape. I've watched a LeeSin juke the enemy team to Death, and a Kayle ult her way to pwn'ing the enemy team. But in every scenario the enemy team made mistake after mistake after mistake that the 1 champion was able to take advantage of.
What you're asking for is already possible. But it requires a great deal of skill. But there will never be a champion able to 1v5 just for being who they are. It will require smart counter play on their part, and lots of mistakes on the enemies part.