In the novel The Forsythe Saga the heroine Irene has a similarly doomed grand love affair with Philip Bossiney who also dies. She goes on to marry Soames Forsythe and we trace her fortunes as well as other members of the family in subsequent years. I guess it depends where we train our focus and whether we stop at one segment of Ennis' life - the Love Affair of which both Ennis and Jack are integral parts - or whether we continue on with Ennis as the main protagonist as he journeys through different parts of his life.
There are a number of fanfics that center around his family relationships over the subsequent years. But if the story is to continue past the point of the original ending, then Jack not being physically present has to be dealt with some way or another. Or there are the alternatives of "pre-quels" (not very common but some out there), fill-ins of scenes and characters not developed (e.g. Randall, the vacation at Don Vroe's cabin only mentioned in passing in the film) or AU. I've read examples of all these and some work, some don't just like in any other kind of fiction.
Sometimes the orthodoxy centers around how crushed Ennis is "expected" to be, and how intransigent his fears and homophobia are perceived as being. The film in particular leaves all that unresolved, as well as the question of how much Ennis is going to be affected by changing attitudes toward homosexuality, or whether he'll be affected by it at all, how his relationships with his daughters are going to evolve over time. There does seem to be a certain percentage of resistence to the idea of his being able to do anything but grieve, drink and feel guilty; and there's a limit to where a plot line can go with that.
And some of that orthodoxy is essentially a reflection of the question: whether that closet door where the shirts are at the end represents a retreat into fear or an enshrining of the relationship and what it meant. If it's the former, then there's the question of how to apply that to Jack, as he had the shirts much more hidden than Ennis does, and that isn't addressed very often.