What I keep coming back to is the Yasuo finding his master's killer lore. How cool would it be to have the killer join the rift? Riven is not the confirmed killer so there can be still room to add one.
It's pretty obvious that she's the killer.
I LIGHT A GIANT BILLBOARD VISIBLE FROM SPACE
"KASSADIN'S DAUGHTER PLEASE I'VE BEEN WAITING YEARS"
Ahem. I'm okay.
I think that if Riven isn't the killer then they'll simply add the killer as a champion eventually,but they won't reveal this immediatly so that we'll have to figure out who of the 2 is the real killer.
