Unique Champion Design - shapeshifting
I don't know how feasible this would really be in a regular game, but I think it would be pretty cool to have a champion whose ultimate ability allows them to temporarily play as a different champion. They could choose any champion they wish, even one already being played in the game!
The spell would probably have something like a 5 second channel time (during which the player could select which champion he wanted to play) and it would be interrupted if you took damage or were disabled. The time you could spend as the other champion would increase as you leveled up R, ranging maybe from 30 seconds to 1 minute.
For champions that use different items, the ultimate could be designed to swap out the player's current items for ones in the recommended tab of the other champion, and then revert back once the ability ended.
This could be pretty cool, as it allows for a larger variance in gameplay throughout the match. It would allow the player to decide which champion would best suit his needs for certain portions of the game. If a top player sees his teammates engaged with the enemy near dragon, he could either decide to change to a strong split-pushing champ and try to take down turrets, or he could change to a strong team-fighting champ and teleport in to help his allies.
The ultimate would probably need to have a lengthy cooldown to compensate for its versatility, and there would obviously have to be a lot of tweaks to make this work. The original champion might be slightly underwhelming in its standard form so that enemy teams have a weakness they can clearly work around.
But I think having a champion with the ability to change comps during teamfights throughout the game would be great. As is, if the enemy team has a better comp, you basically have to hope they make mistakes to win. At best, you can try and have your bruiser champs focus more on damage or tanking based on where your weaknesses are, but you're style of fighting would still look the same. Having a champ be able to change characters for a short duration allows you to try and pick an assassin for an early-game team fight, switch to a tank for another team fight, then to a DPS champ late game, all based on your team's needs at that time.