Kayn idea
What if when hitting the transform threshold for Kayn, Kayn gained half of the benefits of the form that is locked out for 4 minutes? During games where one form is definitely the better choice, getting the other form instead and having to wait for 4 minutes sucks. Allowing Kayn to have half of the power now while waiting for the whole transformation could smooth out the waiting period.
Thematically, it could also be seen as Kayn or Rhast fighting against the impending domination of the form that can be unlocked right away