I think Ahri fits as an Assassin more than a Mage
Mages supposed to have high AoE damage and usually paired with some CCs, that's the main reason mages are picked, for their AoE presence in team fight. I mean, look at Brand, Lux, Vel'Koz and Swain, they offer AoE damage AND CC. Now take a look at Ahri. Lore wise, a gumiho pretty much screams an assassin, they lure a target, kill them unnoticed, run away. Kit wise, she only has 1 AoE spell (Q), W and R is used a lot more for single target burst, otherwise their damage are lackluster. Charm is a single target CC which requires hard positioning. I mean, look at her kit, it brings much less as a mage compared to other mages. Damage?Mediocre. CC?Only single target Charm. AoE? Low. Single Target?High. With her being shifted to mage, she's completely outclassed by other champions on BOTH roles.