If Riot/Tencent decided to make another game in the league universe
Intro: Just a little intro before we get in to it. As I'm posting this it's 5:12am and I'm very tired. Mistakes will most likely be made. It's also quite a read and I don't see how I can condense it into a tldr form unless I do it by section. After I wake up I'll edit all the mistakes, format it properly, maybe expand on it in the comments and maybe add the tldr but enjoy the read and let me know what you think about it.
Main Story: The setting would be something along the lines of all 5 darkin are now out are free at last. Rhaast consumed kayn, varus' weapon consumed him, aatrox(got reworked so he's actually healthy to play) and the others (who we don't know currently) are all together. Them being together once again is a bad omen upon all of runeterra as they bring the end of the world with them. To start the end of the world they release all of the world runes that ryze trapped. By doing this the whole world is out for power now and it's up to you to put them back in to place and stop the darkin. Variation that can happen in this story line would be that the major cities form factions. Demacia, Ionia, Piltover could be one group. Another could be Noxus, Bilgewater, Zaun. The other major cities might fall in to these two groups, create another group entirely, or be standalone cities. These huge factions would fight each other for the world runes. This mostly simplifies the huge confusion that would be if it was just every city for themselves.
Gameplay: The game would play a lot like diablo, unsure if it would just be parties of 5 or more of an MMO style game. Both of these styles would provide a different feel to the game as an MMO would have to be a lot larger than the more local game style. But the local style with small parties would feel more meaningful to each player and league of legends teams can just hop on this game together in a simpler fashion.
Classes: Another big thing for gameplay would be the ability create a champion based on already ready subclasses (picture below). https://imgur.com/a/bgXw5
With this class system you can choose a set of 4 from maybe 12-16 abilities per subclass so there's still a lot of variety in the game.
Items: Talking about variety items would be another big part of gameplay maybe even still having to go get some long swords from smaller enemies to make a serrated dirk to finally make that duskblade(that's so strong right now). While that's more simple terms you would have to grab semi-unique items that would combine together to make stronger items until making a legendary item, these should be unique to this game and maybe have a legendary blacksmith that makes items from league of legends. PVP: This is something I can't put my finger on what to do with it as league of legends is mostly a PVP community they would want PVP. Riot can probably make it so you can port over your champion from this new game into league of legends. Unless the gameplay would feel to different fundamentals wise so they would have to recreate an entire pvp inside of the game.
Misc: Art style should stay the same I don't see why they would change it. We would get a good amount of lore from this game for the lore readers. You would see all the champions you know and love in the game as a quest giver. It would be fun to have Easter eggs like an Arcade that has a machine that you get sucked into and you would have to fight the boss skins from the arcade skin line. Or fighting a secret boss that's rift herald/baron/elemental drakes.
Conclusion: Alright that's everything I can think of right now theres probably more that I'm not seeing right now, if anything I'll post a comment with more ideas. If you did read all of it drop a comment letting me know if this is a good idea. I don't see this or anything like this being made for another couple of years. Thank you for reading!