I'm hoping Xerath, but he was only recently becoming popular, so it'd be a bit odd if they started designing an ultimate skin for him a while back.
But out of all the possible candidates, he has a lot of potential for skin ideas, and he's had a pretty bumpy road with his skins to boot. His on release skins had to be recolored because they all were blue energy with small bits of green or red metal instead of blue, and molten xerath is cool and all, but it's almost like a more orange version of battlecast xerath.
Plus this is getting released very close to a Shurima desert event thing rito has planned supposedly, and Xerath was a mage of an ancient civilization of the desert.
I'm just hoping I'm not deluding myself for the sake of a dream.