You know... I could kind of see an Elementalist Taliyah.
The ultimate versions in most cases will still be better as they are a 3250 skin and come with more features. Ezreal is just severally outdated, so he won't win either way. There are still plenty of Unique skins in the game though, things that I doubt we'll see with the same names even if they follow the themes. Things like Forcast Janna, cannot be easily replicated on to others.
No one has ever really said Ultimate skin were meant to be a unique theme for a champion. If they have, where is this statement? For all we know this years ultimate skin or future ultimate skins could be part of an existing theme as well.