I think it would be good for players if Riot added a "Sandbox" mode

I think it would be fun if players could test builds in a custom type game where you just start the game with unlimited gold so that you can test builds, instead of having to go through the entire game just to test 1 different item, or something of the like. If players were just able to play a mode that wouldn't count for wins or losses, and was used for the purpose of build or mechanics testing, it would allow players to better develop builds since they would better be able to test different items, rather than having to go through the length of an entire game. Also, what if the team they are playing against starts beating them, and the whole reason the player joined a game was just to test a build and now they don't get the chance to? It's just a way to keep the players ideas and minds and could be a step in a positive direction to allow for further player development.