When Rose Tyler accessed the power of the time vortex she resurrected the deceased Jack and (unintentionally) made him immortal, a fixed point in time apparently, perhaps even a paradox considering he died and she defied the laws of time & space to restore him. There's a similarity here to what happens with Clara Oswald...
I don't know a lot about what's been revealed about Jack in recent years, but given the rationale Russell Davies gave he might eventually evolve into being The Face Of Boe, or... what? Really if he is literally immortal and sits outside the effects of entropy then by definition he can't evolve - so can't possibly become Boe. So again, in theory wouldn't he be around in virtually every story we have seen that is set in the future? Somewhere Jack is living and travelling in the future's of New Earth, somewhere out there in the Universe perhaps?
Just what DID become of him?
Who can say.
So there will come a time in future history where just Dr, Jack, and Clara exist period?
What about that viking girl who got that tech in her from Dr, is she also immortal?