Minion Skins, do I need to say more?
I'm not sure why Riot hasn't thought of this before, pool party minions shows they can do it, it would be a great addition to the game, Some general idea's I've thought of on the spot.
Blood Moon Minions:
Prehistoric Minions
Definately Not Minions
Poro Minions
Heart Seeker Minions
The only thing I can think that would stop them doing this technically is that having everyone using different minion skins in the same game could cause some issues.
If this was the case and you couldn't have multiple minion skins: What if one player who had brought the skin pack could 'suggest' this skin in a pre-game lobby, the other members of the lobby (ally and enemy team) would then see all the suggested minion skins and vote for the one they wanted for that game?
Just my two cents.