While 20th Century Fox hasn’t officially announced yet which of the X-Men movies in development will follow X-Men: Dark Phoenix, the studio has set several dates aside for its upcoming Marvel releases, including June 7 and November 22 in 2019; March 13, June 26 and October 2 in 2020; and May 5 in 2021.
Both November 22, 2019 and October 2, 2020 would be especially good times to release a direct Dark Phoenix follow-up, although given that Dark Phoenix doesn’t have an extended production, the latter date would be the likelier option.
We have our fingers crossed that Dark Phoenix will do a better job playing with the Dark Phoenix Saga storyline than The Last Stand did, and if this turns out to be a duology, that would ideally increase its chances of success in the long run.
Some of the projects attached to these dates are easy to predict, like X-Force and Gambit, but it’s not unreasonable to assume that another main X-Men movie might take one of these slots.