Old School League
I think Riot should implement a feature that allows players to play patches from previous seasons, at least in custom games. Sure, it would require a huge amount of technological work, but its absolutely doable. Even if it took 5 years to create such a feature, it would be revolutionary.
In my opinion, League is the best MOBA game currently available, and has been for some time now. However, I think previous versions of the game are far superior to the current version, even with the outdated graphics and unpolished gameplay. Whether you personally agree or not, many players have said the same. Plus when I sit down to play league, I don't want to have to slog through patch notes just to figure out what the game is now. And if I think Riot made a poor design decision, my only options are to deal with it or quit. Its safe to say that a huge amount of players would appreciate being able to know they're playing a game they can enjoy without it constantly changing into something unrecognizable.
It would make sense to have the matchmaking service only available for the current, most recent patch, and have this feature available only in custom games. It would also make sense to allow players to pick and choose which patches to download in order to avoid having the game be 100gb.
Many people will argue that this will split the community. I disagree. If anything, it would expand the community, and would show that Riot is no longer okay with leaving the older players in the dust (RIP Dunkey). Sure, it might bring players away from the current patch, but it wouldn't if the current patch was more enjoyable than previous versions of the game. Riot's assumption that all work is good work will be what eventually kills League. If they allowed players to have access to the game they enjoy, it would show they're more concerned about their community having fun (which, as a video game company, should be their first priority).
"But what about the competitive scene?" Again, it might decrease the popularity of the current patch, but this wouldn't be a problem if the current game was better than the older versions. Plus pro players constantly point out how stressful it is to have to constantly be learning how to play a brand new game at the highest level every month or so. And how exciting would it be to see veterans being able to dish it out again in the versions of the game they learned? This feature would only expand the pro scene. There is an exciting aspect to seeing players compete in a constantly evolving game, but its unbelievably risky and stressful to create a lasting and stable career in such an environment. In my opinion, Riot should continue hosting Worlds and all related competitions themselves for the live version, but allow other organizations to organize their own tournaments for older versions of the game.
This feature doesn't even have to be free. Allowing the live version to be free is generous enough imo. Even if they charged a one time fee of $20, they could easily become the richest video game company in history. In 2014, Riot made a statement showing that League had over 67 million players active monthly. If even just 1/4th of those players purchased the Old School feature, that would result in $335,000,000 for Riot. I think that justifies the work that would need to be done in order for this feature to be created. They could even hire a bugfixing team to constantly focus on fixing the older versions, while keeping the gameplay as it was.
The way I see it, this feature would allow League to take over the world again.