I'm leaning towards either
(1) Dexter + Son on the beach in some foreign land, them having escaped and his dark passenger put to rest
(2) them doing a time-jump thing like wot they did with Harry Potter (iiuc) but in this case they jump some 20 odd years and we find Dexter all wrapped up just as Dexter Jnr stabs him
(3) Dexter and Deb driving alone, stop to admire some pidgeons/doves and then both dying in a hail of bullets from the meanold Sheriff and bunch of towns folk.
or (4) Dexter dying after being nailed to a plank of wood for 3 days.
or something else.