Could be really cool. Making the splash art for such skins would be a massive undertaking, but the skins/models themselves could just use a filter or swanky effect to sort of unify them across all champions.
Maybe something like Smite where the heroes (gods) are just solid gold, maybe have some cool ghostly effects, maybe make them look like they're made of chrome steel.
Also - there's the problem of difficulty in actually unlocking those skins. Champions like Yi and Katarina can fairly easily score a penta kill, where champions like Janna or Braum - good luck with that.
There's also a minor issue of maps and modes for earning and sporting the skins, as earning a pentakill in Howlyng Abyss is a lot easier than getting one in Rift. Or getting one in co-op is substantially easier than Abyss. Earning a pentakill in Treelines is subsequently the most difficult, as you'd need to time it perfectly with resurrections, and probably need to be fighting opponents with Revive.