Well im one of those people who have all the champions (i dont have all the champs, but i hate the 3 champs that i dont have :P) and with that said i have lots of ip in my account that i cant use it anywhere exept from runes and the 3 champs that im missing... So how about this ideas Riot:
My first idea is that every person that has all the champions, can get a hextech key or chest for 5000 per, with a daily limit, so people who have 50000+ ip cant spam getting chests/keys
My second idea is that you can make a system that makes people who have all champions exchange ip for a skin on champions that they have level 7. For example i have Katarina at level 7, I go to the store and i can buy any skin i want for katarina just for ip. The ip price is based on the real price: example 950 rp = 15900 ip, 1350 rp = 23150 ip etc...
I hope that one of those ideas will be available in the future cause people who have ip cant spend it anywhere.... and we need to do something about it riot!