I dislike the idea of playing non-champion characters such as skins or items or minions or whatever else. And even moreso, I thoroughly disapprove of playing canon Riot characters that hold key positions in the Institute of War. Don't forget that all those characters have Riot employees behind them, and basically grabbing their canon characters out of their hands would be a grand failure of RP etiquette. Also, basically none of those characters have definitive bios that help a player outline their course of behavior, so any kind of portrayal is almost bound to be out of character.
I left the Runebook roleplay when people began roleplaying as Pink Ward and Left Jungle Wolf On the Blue Side Of Rift. As exciting as that sounds, it just does not work out, so please... don't.