Polymathy

you must take this feat before you have gained a new class feature from your subclass. when you advance to a higher character level that unlocks a feature in your subclass, you may forgo taking that feature and substitute it with another subclass feature of the the same class. you cannot substitute a subclass feature from a character level you have not yet reached. 

this feat cannot substitute a feature that improves on a feature that you don’t already have (I.e. you cannot substitute greater portent as a 14th level wizard if you don’t already have portent.) 

this feat can only be used once per sublclass feature substituted, but can be chosen more than once.