An interesting idea, but only on paper. The difference between the vs. event and possible future events is that this is it's own separate thing from the canon universe. You could argue that this is the very beginning of the world, but even so it doesn't mean it has any significance to today's canon lore since this would have happened so long in the past.
And because it's not tied in to the current lore or world, Riot can have the player base decide how the outcome of the story will be since it will be it's own thing. Having something like this in a Noxus vs. Demacian event wouldn't work because, as Mira speculated in one of her posts about the vs. event, people don't want good to win since this is the internet and good is the evil here. People like to be rebellious and chaotic here since it gets them away from the orderly world of reality dictated by their parents and so on.
Because of that, I would almost guarantee that Noxus would win, either due out of more popularity or the fact that they're viewed as more chaotic. As well, this would heavily influence how Riot writes the current lore and events of League's world and would take away much of the control they have over it since it's being decided by the player base, while the vs. event is OK since it won't affect how they write the canon lore.