Maybe - and I'm talking through my hat here - the next one will be "Nathan Drake, these millions of dudes and I will shoot at you if you don't go out for the vague possibility of this treasure we want", and then he has to balance the fact that he's having a lot of fun going out for the vague possibility of treasure with the whole under-duress and possibly-being-shot-at things. Then he will meet Joel and Sully will give Ellie a ride in a helicopter.