Riot, you stated that you needed to work on the game's engine

This game is based around DirectX 9 still, yeah? That means you'd basically have to rewrite the engine to get too much done, and you've stated you kind of need to do that. If you do this, I think it'd be best to choose OpenGL because it could make running LoL on Linux easier and also serves as one step towards a Linux port. That, and OpenGL is just...better right now.