When Square Enix first mentioned an NX version of Dragon Quest XI, they initially backtracked on the idea and said that they were merely "considering" it. In an interview with Nintendo Dream, Yuji Horii has mentioned this version is indeed in development. Not only that, but he wants it to release simultaneously on all three platforms.
Junichi Masuda from Game Freak and Masahiro Sakurai are also in the interview.
This means both Dragon Quest X and XI are in development for NX. As a side-note, Dragon Quest Builders seems likely as well. Here's what producer Noriyoshi Fujimoto said at Gamescom:
Junichi Masuda from Game Freak and Masahiro Sakurai are also in the interview.
Read much more at Gematsu.Masahiro Sakurai: Well then, I would like to ask about each of your new games. First, Dragon Quest XI. The 11th installment in the series will be released simultaneously on Nintendo 3DS and PlayStation 4, and including NX it’s coming out on three platforms, right?
Yuji Horii: That’s right. The use of hardware is different for each system, so it’s quite a challenge.
This means both Dragon Quest X and XI are in development for NX. As a side-note, Dragon Quest Builders seems likely as well. Here's what producer Noriyoshi Fujimoto said at Gamescom:
When asked if it would be a possibility that the upcoming Dragon Quest Builders would receive a port for the Nintendo NX, Fujimoto noted that “Square Enix is aware” of the compatibility between the Dragon Quest series and Nintendo’s consumers.