I was just thinking a cool new mode to add in would be an all random mode, where you are placed in a team and all champions are randomly selected after the game begins.
It is a mode I personally would paly all the time because my summoner lvl may only be 22 but there is really only maybe a dozen champions I have actually played, it would be an exciting mode because you wouldn't know what champion, or what role you will play until after the game starts! As for masteries and runes you could probably just remove the use of them for this mode, or have 5-6 preset types for each champion that riot has chosen, so that when you find out what champion you have you can select what kind of runes you wanted to run out of what they suggest.
To try to make as balanced teams as possible, Riot could make a filter for when the summoners are selected for a game, each summoner is secretly assigned a chamption type (tank, support, bruiser, mage, etc) that way you don't accidentally have a team that is completely AP heroes.
I feel like it would be a really exciting way to play different champions without having to wait each week to see what is free, and would make some really interesting teams. I personally am tired of constantly seeing the same heroes being played (Katarina, Akali, Graves, Sion, Sona, etc)
I mean, wouldn't it be neat to every once in a while see Evelynn, or Urgot? Or play a role you never play?