Okay, I love the concept of the versus event. I think it's amazing, and I would love to see more events like it. However, I feel like choosing Riven, a highly technical champion who isn't super well loved in silver/bronze, and Yasuo, who is one of the most banned champions in League history, might have been a teensy bit of a mistake.
They are two incredibly popular champions who are also somewhat of rivals in the lore. They both sell skins well too.
it's not that dumb.
lore wise it makes sense
both extreamly popular champions across the player base
sej kass akali LB all beat yasuo ban rate as they were banned accross all elos whilst yasuo is mainly banned more the lower you go.
While I do think it's interesting that the two main champions involved are perhaps infamous among the player base, none of the free missions require the use of those two champions, and even the paid ones don't really require any really skill on either of them (and coop vs ai and blind pick work for most things too, removing the banning issue).
As such, it basically could have been any two champions as far as gameplay is concerned, but the thematic for yasuo/riven probably is more exciting than many alternatives. (Some exceptions of course, but theres likely to be more events along these lines in the future so we may see some other VS events with other strong thematic pairings.)
