Pre-Shadow Isles Hecarim
So I was thinking. Since we have pre-void Kassadin, why not have a pre-shadow isles Hecarim skin? This could give Riot a chance to develop the back story on Hecarim more and explain how he came to be the "might of the shadow isles."
The skin could look like a "regular", living, centaur out of greek mythology, Narnia, Harry Potter, Percy Jackson and the Olympians, ect...
Alternatively Riot could work a skin like this in with the Prehistoric/Beast Hunter line of skins.