Custom game options
Custom games aren't very customized. It's just the same as playing a non-custom game except you can adjust team size.
Good for practicing one mechanic, except you still have to go against at least one bot if you want to practice by yourself. There's no way to customize it for endless farming, or to set bots for specific roles (e.i. more than 1 mid), or to customize spawn timers, among other things that could be useful for practicing specific mechanics.
So, it's not optimized for practice in unique settings. It's not optimized for fun either. Why can't custom games have unique settings? This would greatly enhance the quality of life within the community and add years to the life of LoL. You could even make it paid (x amount of dollars per month or year or w/e to make custom games, but can invite non-paid members).
Options could include the ability to customize aspects of the game to a positive or negative value: Movement speed No minimap Start with unlimited gold, to quickly and easily test multiple different set ups Start @ level 18 to quickly and easily test mid game/late game set ups Disabled auto attacks Minion strength/stats/amount of minions Turret strength/stats Or even some weird stuff like infinite range on all abilities, no gravity (flash would shoot you across the map), Funky AI pathing (minions zig zag across the map or something), etc
This would also encourage people to utilize their friend's list more effectively, and draw in new players that aren't interested in a hardcore ranked pro-scene experience or can't achieve that level of skill (most players).