An authorized follow-up is in the works, but this is how I imagine it:
Bates & Anna live happily ever after.
Mary & Edith have a knife fight & both die of tetanus.
Sybil joins the VAD.
Branson returns to Ireland & is killed in the Easter Rising in 1916.
Thomas takes advantage of his medic status to start an under-the-table trade in morphine.
O'Brien, racked with guilt about the miscarriage, has a change of career and becomes a nanny.
The house is turned into a rehab place for wounded soldiers for the duration of the war and the early aftermath, with Mrs. Crawley in charge. Mary & Robert live in just a couple of rooms.
Matthew & Michael die in the war.
The entail is so messed up that it can't be sorted out. Upon the Earl's death, Downton is donated to the National Trust and becomes a tourist destination.
Bates & Anna live happily ever after.
Mary & Edith have a knife fight & both die of tetanus.
Sybil joins the VAD.
Branson returns to Ireland & is killed in the Easter Rising in 1916.
Thomas takes advantage of his medic status to start an under-the-table trade in morphine.
O'Brien, racked with guilt about the miscarriage, has a change of career and becomes a nanny.
The house is turned into a rehab place for wounded soldiers for the duration of the war and the early aftermath, with Mrs. Crawley in charge. Mary & Robert live in just a couple of rooms.
Matthew & Michael die in the war.
The entail is so messed up that it can't be sorted out. Upon the Earl's death, Downton is donated to the National Trust and becomes a tourist destination.