Japanese studio HexaDrive has revealed that it'll be co-developing Dragon Quest XII: The Flames of Fate alongside Square Enix. HexaDrive has worked with Square Enix multiple times in the past, the most notable collaboration being Final Fantasy XV.
A bit of background: HexaDrive primarily works with other companies on fairly large projects. For example, it helped develop The Legend of Zelda: The Wind Waker HD with Nintendo. As such, it's unlikely that HexaDrive has much creative say over Dragon Quest XII — it's probably been brought on for technical purposes.
In any case, HexaDrive is currently recruiting for Dragon Quest XII, which suggests that the finished product is still a long, long way off. Square Enix previously said that the story is done — but it sounds like the game itself is only just getting started.