Since 2011, the only year in which there was no Riven skin is 2013. Ever since 2013, she has gotten a mythic skin (the whole shebang with particles) every year: DragonBlade Riven in 2014, Arcade Riven in 2015, Championship Riven in 2016, Dawnbringer Riven in 2017 and now PulseFire Riven in 2018.
Following this trend, it means next year will be Conqueror Riven, or it will be a Winter/Summer/Halloween/Valentine skin. Some skin that's not sold all year 'round.
This isn't a "bash Riven" thread, since I understand she's a popular champion with a lot of diehard mains (who die a little too often for my tastes), but it feels a bit sour for those champions who have to wait years to get a new skin.
Most of all, this is a thread you can quote a year from now to see if the prediction held true.