How about a champion who steals the abilities from those he kills?

A champion who has the passive of stealing the abilities of those he kills, not assists, but kills. he could be something created by victor and unleashed on the enemies of zaun...

As a rule he will not have an ultimate, but will have two stances he can toggle between, a melee form with a chainsaw, and a ranged form with a machinegun

the catch? all abilities which deal true damage will deal magic damage instead, and all magic damage will become physical damage.