This will not be Drakes last game, mark my words. SPOILERS
SPOILERS-
All if these games have been stellar. Theres way to much money to be made.
Aside from that-
Nadine escaped. So weve got her running around out there.
Drake and Elena have a daughter who seems to be walking in here parents footsteps.
The daughter is the one factor i can see that would mutually motivate Nate and Elena to voluntarilly go off on at least one more adventure....maybe a kidnapping to force Drake out of retirement?
And finally, theres $400 million sitting at the bottom of a lagoon in a cave.