According to my research the game already has 2 separate brackets per CL per 1000 people signed up for a story event. A newbie or poorly playing bracket and a vet bracket. Just create a third bracket per CL. If the average level of your top 3 characters equals a specific number say over 255 for CL 6 you are placed in a bracket with only those above this number. No need for locking out characters or basing it on Shield levels. Everyone wins, those below the number don’t have to lose to people slumming but those that want to drop for easy fights can do so with out penalty.
(I apologize if my research information is outdated)