CHAMPIAN MASTERIESSSSSSSSS !!!!!!!! PLEASE READ !!!!!!!!!
Ok so I don't know if Riot actually takes this seriously but I hope they do. My idea is to just set a tab on all the champions in your owned champion page where you can set the mastery tree for them. That way you dont have to go through the pain of choosing a mastery tree every time you play a different champ. They should still have the option to choose custom masteries but this will leave those 20 mastery slots for much more customized trees for counters and if you dont have a very specific tree in mind then itll be fine you already have it set.