Top : Trundle ??? Maybe Fiora. IDN , this year there wasn´t really a top laner that took total control of the meta
JG : Nidalee has a massive Pick/Ban ratio all across the world. Reksai and Gragas also are pretty big names
Mid : Viktor has a pretty high pick ban ratio (The highest in EU and LCK), so she is a massive candidate. However , Vladimir acually has a higher pick/ban ratio then Viktor , so he is also a massive contender. Cassie really has shown up recently , but Riot hates her and her meta take over is kinda recent so I doubt that it will be her
ADC : The role that I doubt that will get the skin , since Sivir got it last year , but if I had to choose , I would go with Ashe.
Support : Karma is a massive contender , being the 4 most picked/banned champ in NA , the 3 in EU and the 6 in the LCK. I could see her getting the skin. Bard also could get the skin , but a lot like Cassie , he is a underdog.
Summing up my thoughts , Viktor , Vlad , Nid and Karma would be my bets.