"I'm not sure if this is something that's been discussed anywhere else, but I was watching the series and noticed how often the characters reference grief, loss, and death and — when watching all movies back to back so far — it appears that the plots surround each of the stages of grief.
John Wick — Denial
John’s living by himself with his wife’s things left around the house, untouched, as if she’ll come back to use them. We see him trying to live normally and act like nothing's wrong, but we see him in anguish. He's even in denial up until the end of the film as to whether or not he's 'back.'
John Wick: Chapter 2 — Anger
His killing spree from the first movie entices Santiago to pull him out of retirement for good, and to use and betray John. John's rage gets the better of him and he kills on Continental grounds at the end of the film.
John Wick: Chapter 3 — Parabellum — Bargaining
The length of the film is John begging his connections for help. He asks the doctor for meds, the director for a ticket, forces Sofia’s marker, seeks guidance from Berrada, and even walks through the desert to find the high table to beg them for another chance at life.
So, my theory is that there will only be two more movies (which have both been announced) and they'll explore the final two stages:
Upcoming Fourth Film — Depression
The fourth movie will probably exist around the depressive stage of grief where John is forced to stay in the life that he left. He’s become an empty shell, slower and sloppier.
Upcoming Fifth Film — Acceptance
Whatever his choices may be, the final stage of grief could be John finally coming to terms with who he is and continuing as an assassin, dying, or taking his dog home for good, this time a better man."