As in bring Cox back, have Braff play a persnickety condescending middle-aged JD that has become like his mentor because the medical profession has done that to him, or have Braff play Cox?
I like the middle one, we can still have Cox present but maybe he's on the board and semi-retired now. I think the character arc of JD becoming bitter like his mentor over the years could yield some good stuff.
Judging by how these things usually go, cox would become Kelso and everything he hated about him, braff wpuld become cox and everything he hated about him, and timothee chalamet or some shit would be the new JD but all angsty and not funny.