Thank heaven for fan fiction

Option 1: John's retirement and the end of the show

Pros: Logical; and surely they cannot find yet another "cousin Barnaby"

Cons: Why kill the goose that lays the golden egg

Intriguing possibilities:

(1) Going by previous track record, the "new DCI long-lost cousin XYZ Barnaby" could be played by the bloke who was the baddie flirting with Sarah in s15e2 Murder of Innocence. If they do this, and then bring DI Jones back from Brighton on a case, ahem... good times might be had by all!

(2) DI Winter as lead? Sorry Jamie baby, not sure that's gonna work. I mean it's time for you to fly the nest and all, but somehow, you taking on the murderous denizens of Midsomer without the stabilizing influence of a DCI Barnaby just doesn't cut it.

Option 2: John's promotion to Superintendent and the show goes on

Lots of intriguing possibilities here, mostly hinging on John now becoming a desk-warrior and bringing in one of the ex-sergeants as DCI:

(1) DCI Gavin Troy: Now happily (and FINALLY!) out of the closet, and determined to drag his DS (Winter) out willy-nilly

(2) DCI Ben Jones: Ties up DS Winter in all sorts of knots without even trying... just existing in the same room is enough. To add to the fun, his by-now centurion Gran and Fleur become besties, interfering in his love life and his cases alike, much to his embarrassment (to say nothing of Jamie's) and Barnaby's chagrin

(3) DCI Dan Scott: Wild boy turned good cop and boss... walks in determined to guide DS Winter through what he considers a much-overdue promotion, which that poor idiot willfully misinterprets as "new boss hates me" despite John's (and Sarah's) best attempts at mediation. Plus he ends up with a massive crush on the new boss, reminding Sarah of her 6th form students. And all the while, Scott is happily married to Cully Barnaby who finally got around to giving Simon the heave-ho some years ago!