Andrea Dovizioso, runner-up in the championship for the past three seasons, announced in August that he would not be staying at the main Ducati team beyond the end of the season.
The Italian has yet to detail his plans for next year.
Bagnaia and Miller, whose move was announced in May, are currently team mates at Pramac.
Double Moto2 champion Zarco, 30, will leave Avintia Ducati to team up with promoted Spanish Moto2 rider Jorge Martin, 22, in a fresh pairing.
Martin, the 2018 Moto3 champion, was a strong contender for the Moto2 title until he contracted COVID-19 and had to miss two races.
Bagnaia, 23, has had an impressive year despite missing three races due to a fractured tibia. The 2018 Moto2 champion returned to finish second at Misano on Sept. 13.
Avintia are expected to announce Italian Enea Bastianini as Zarco's replacement.
Ducati said all four official riders will ride the latest version of the Desmosedici bike with the same technical specifications. (Reporting by Alan Baldwin, editing by Christian Radnedge)