Somewhat +1. The current levels between 60 to 64 are fine, if an area is made with permanent access similar to Cerulean Cave with higher levels, people will very probably abandon it for the latter. I'd say each region should have side optional high level zones from 60 to 64 with evolved forms that you unlock through quests with different pokemon types. Example:
Johto - Bell Tower converted into a Flying Type area with level 60 to 64 pokemon locked behind a quest. (The Celebi one for example, idk, I would give Burned Tower and Bell Tower some usage. Maybe even Ruins of Alph having a secret 60 to 64 area with Psychic types)
Hoenn - Water type cave area in Route 134, similar to 3rd gen where the Regi Trio puzzle is located.
Sinnoh - Snowpoint Temple - Ice type area with level 60 to 64 pokemon.
All with 2nd to 3rd stage pokemon similar to Cerulean Cave. Cerulean Cave could also get slight reworks to be honest.