So who's next riot?

Aighty then, so the new MF ultimate skin just came out, and I gotta say, love the theme to it. But now I can't get this one thought out of my head, who's the next champion due for an ultimate skin? And is there some rule to making them? I do believe they're targeting older champions though. But I can't go completely by that since lux isn't that old along with ezreal. So who do you guys think deserves an ultimate skin next? Katarina? Ashe? Anivia? Anyone besides zed or Yasuo?