It bugs me alot that we just have 20 mastery pages. Masteries are there for fine tuning of your character, so even if there are champions with similarities, there could be you want to differentiate the masteries by a small tweak. But changing these in champ select and always remembering how you want to change it is kinda hard, so I have a suggestion for the mastery page system.
Give it the same layout like current item sets. You are able to create as many mastery pages as you want and tie them to specific champions. In champ select, after picking a champion you just get shown the mastery pages who are tied to the champion you picked. So that would even reduce the load of mastery pages in champ select, since you don't see the complete list of 20, but just the mastery pages you are actually using for the actual champ you are playing.
Only problem I see is that you can't buy a champion and jump into a game immediatly, using the mastery page of another champion, since you have to tie mastery pages to that champ first or you won't have any. Maybe they could give you a remind to tie a mastery page to a champion after purchasing? Or we can find another solution together?
I would like to hear opinions about that topic.
Edit: Solution for newly purchased champions: Riot could create recommended mastery pages, which are intact if you didn't tie a page to the champion yourself. Just like item sets.