I really like the Light X L pairing, but I find it kinda hard to see. From what I can tell from the manga and anime, both Light and L seem that they would both be an agressive Seme. Depending on who is being portrayed as the uke and whos the seme, I don't think that Light would be the uke because of his superiority complex, and I can't see L as the uke because hes too agressive and doesn't like to lose.