The fourth of the new X-Men movies is now confirmed for November 2, 2018. Presumably it will star Sansa Stark, reprising her role from Apocalypse, as the Dark Phoenix. Ottman scored Days of Future Past and Apocalypse so I wouldn't be surprised to see him back, but I guess we'll find out.
If Bryan Singer is returning to the X-verse again to direct this film, chances are he'll bring Ottman along as well. Though it'd be a bit ironic if Singer were unable to direct this one; it'll have been the second time he'll have missed out on telling the "Dark Phoenix" story (the first time was X-Men 3 of course).