Square Enix has confirmed that they have started working on a brand new mainline Dragon Quest game. They did the same with Dragon Quest XI in June 2014 and the game was released in Japan on July 2017.
During the announcement for a new mobile game called Dragon Quest Walk, Dragon Quest series executive producer Yuu Miyake confirmed (via) that they have started working on a new mainline game.
By the way, about Dragon Quest XII, which I’m sure is on everyone’s mind—[Yuji] Horii and I are currently making preparations on series developments that will connect to XII. I think it’s still too early, but we would like announce it in some form, so please wait a little while longer.
Dragon Quest XI was a brand new mainline game that was released on the consoles and PC for the first time in the series. It was developed on Unreal Engine 4. It is possible now that they have an engine ready for a brand new Dragon Quest game, it will take them less time to develop it.
Dragon Quest XI was also released for the Nintendo 3DS so it remains to be seen what kind of plans Square Enix have in store for the upcoming mainline Dragon Quest game.