An Assassin/Tank would be a broken antithesis. Assassins are categorised by high burst damage, with high mobility, low CC (apart from a select few) and no innate tankiness apart from the odd shield in cases like Ekko. Tanks are generally CC monsters that don't die easily. Assassins are never meant to be tanky. Whenever a tank assassin build rolls around, Riot is generally quick to nerf it into obscurity.
Ezreal, Kog'Maw, Kennen and Azir are all marksman/mages. You could also include Varus in that because he can play like a mage, focusing solely on his abilities rather than his auto attacks.