I know Riot mostly has to focus around balancing League of Legends and the new content that has to come with it, but it's one of the biggest gaming companies and I'm sure they could have another team manage a brand new game, which would be an MMORPG.
I think this would give the game a way bigger fanbase, as opposed to its current almost non-existent fanbase (fanbase and playerbase are two different things), because most players don't care too much about the lore or the world- this is because almost none of League's lore is represented in-game. This way, we could explore it in-depth.
You'd get to choose between Ionia, Zaun, Piltover, Demacia and Noxus as starting areas and each would have unique classes, with Bilgewater, Targon and the Shadow Isles being optional factions you can join later on. Each would have their own storylines.
Some lategame dungeons being portals to the Void, where the bosses are Cho'Gath and the like. Some of the in-lore characters could be NPC's, with Aurelion maybe even popping up, who knows?
The amount of customization and character creation all the different factions would offer, especially also in terms of abilities, would be amazing. I think this could quite literally be the greatest MMORPG of all time, honestly? The amount of genuinely good content seems endless with League's universe, honestly. From sailing to cooking to forging to just looking for artifacts, there's more than enough potential in Runeterra for something great. I think everyone'd love to explore it.
Plus, wouldn't you love exploring Piltover and then making the descent to Zaun?!
EDIT: Think of all the PVP options as well with the Demacia/Noxus factions and Ionia/Noxus, all of that stuff with the warring factions. Plus Riot's Alternate Universes could be events to sell outfits, like a Star Guardian event for SG outfits!!!
Think about it. It'd be godly.