Mirror champion

After realising of Kayn, i tought of a champion concept. Mirror champion or something like that. I tought only about his ult. How would be a champion to have on his ult, the ability to be for 10-15 seconds, the last champion that he had hit, but dealing 25-50-75% of that champion damage on abilities and basics. (without ult, of course) For example, this mirror champion had a duel on top with a ww. He won the duel, and sees that on mid the enemy mid laner is low hp. He uses his ult and transforms into ww and gets that extra movement speed to roam down mid after that low hp champion. Of course, the idea is simple to be said, but i know it would be a hard work to do and balance a champion like this.