How would you like to see it all end? Mass Effect has wiped the slate clean, so one can assume at one point Dragon Age will. I for one would like to see an endgame that brings back all PC's under player control and has them working in different parts of Thedas towards stopping the final Blight, which has two archdemons, and Solas, who I imagine will resort to some drastic measures to restore his people. It would be cool to see powerful factions like the Inquisition and Wardens struggling over limited resources while still trying to work together, and the political jostling to see who comes out the true winners in the end. Followers with more than one loyalty (i.e. Varric) would have to be won over by one faction or another based on their actions. It would be awesome to see the Inquisitor working with/against the HoF/Warden Commander, Hawke (if alive), and the PC from the next game.
So, how does this work if both the Hero of Ferelden and Hawke are gone?
For that matter, let's assume that both the HoF and Hawke are dead, and the following characters are narratively unavailable due to either death or lack of loyalty to any major PC:
Alistair: Dead in Origins or in Inquisition
Zevran: Dead in Origins
Wynne: Dead (either in Origins or the novels later)
Shale: Dead in Origins
Sten: Not recruited in Origins, thus has no reason to be present.
Dog: Dead (non-Cousland origin)
Loghain: Dead in Origins
Oghren: Refused the Joining by an Orlesian Warden Commander in Awakening, or left party in Origins due to low approval.
Nathanial: Dead in Awakening
Velanna: Dead in Awakening
Sigrun: Not recruited in Awakening
Anders: Dead in Dragon Age 2
Justice: Unavailable after Ander's death.
Fenris: Not recruited in Dragon Age 2.
Bethany/Carver: Dead in Dragon Age 2
Sebastian: Not recruited in Dragon Age 2
Merrill: Dead in Dragon Age 2
Isabela: Not recruited in Dragon Age 2
Iron Bull: Not recruited in Inquisition
Cole: Not recruited in Inquisition
Sera: Not recruited in Inquisition
Dorian: Not recruited in Inquisition
Blackwall: Not recruited or dead in Inquisition
Vivienne: Not recruited in Inquisition
So...that leaves...?
Morrigan - who may be unavailable or not loyal to the Inquisitor.
Leliana - who may be the Divine if it is not Cassandra (Vivienne was not recruited, thus she is not Divine)
Cassandra - who may be the Divine if it is not Leliana (Vivienne was not recruited, thus she is not Divine)
Solas - likely unavailable for obvious reasons
Varric
Aveline - loyalties may be conflicted if Hawke is dead
And that narrows it down, without using unexpected survival of characters assumed dead, or unexpected loyalty to the Inquisitor, to:
The Inquisitor
Varric
Cassandra or Leliana (but not both)
Aveline
Morrigan
That's all we can guarantee, unless Solas is somehow available.
I know that's somewhat extreme, but it is nonetheless possible.