What you want to describe is you want cosmetics for free by blindly paying a set amount each month instead of physically controlling your money. That isn't a hybrid that's a subscription with a glorified trial.
RIFT and TERA model is the game is 100% free (no content is behind a "paywall") and there is cosmetics you can buy in the store. Since you want to pay $15 a month anyway you can still do that it's just you get to choose what you want.
Basically you want a subscription with a trial and you give them money blindly and hope they give you what you want instead of you picking exactly what you want.