What if the Institute of War is yet to be created?
Yeah, I know the chances for this are ~ 0, but it got me thinking.
The state in which Runeterra is now, it's pretty similar to the periods of Rune Wars or between them. Riot said Summoners still exist in some way now. We know from the old lore that Summoners existed even before IoW, as war mages who worked as mercenaries usually. If Valoran is not stabilized now by any supranational entity, there's danger of grand-scale wars.
One more thing is interesting, because if there is no League of Legends, why is the game still called that way? Perhaps it will be created in the future, but that moment is at some unidentified moment in time in the future; until then, a lot of lore could be thought out.
The new IoW could be formed to behave just as we wanted to perceive the old IoW: not as an all-powerful organization that keeps Champions in the Building forever, and controlling them like puppets, but as barely stitched-together institution that organizes matches whenever two nations decide they want to settle a small dispute on the Fields. IoW would still try to prevent large-scale wars, but not with all-powerful magic, but by desperately trying to appeal to reason, using examples of past Rune Wars; using tricks or quick fixes, attempting shady executions of most war-wanting individuals. Of course, the chances of their failure is great. But there's still more drama and suspense in: "will there be another Rune War? Because the last one nearly ripped the entire planet apart, we must prevent it", than when nobody cares about anyone, "Let's go to war!! Yeaaaarrr", and everyone just jumps in random wars for which we don't know how dangerous they might be.