The ITV drama, telling the story of the aristocratic Crawley family, is set to return to screens later this year for a fifth series, having made its debut in 2010.
But Lord Fellowes said the saga would not "go on forever". He has been commissioned to write a new US drama called The Gilded Age but said work on the show would not start until Downton had come to an end.
Lord Fellowes said: "It will happen when Downton finishes because I just couldn't do both at once.
"I haven't written it yet, but it's about the old aristocracy, the Winthrops and the Stuyvesants and the new money of oil and gas and shipping in the 1870s. It will all be fiction - it won't be real people - but when those families descended on New York, they took over."
The writer reassured fans there would be a fifth series of Downton but referring to a long-running US courtroom drama, added: "I don't know yet if there is a season six, but it's not going to go on forever. It won't be Perry Mason."
He also speculated that if the Crawleys had been a real family, they would have still be running the estate today - albeit in a more "businesslike" way.
He said: "My own belief is they would survive, but they would be living in a back wing - and you could buy a ticket to visit and they would only come out in the winter."
Downton Abbey was last seen on British screens on Christmas Day, when a special episode was broadcast showing the family visiting London for debutante Lady Rose's coming out.