Those fine folks over at ResetEra have discovered job listings for a Legend of Zelda related project, seemingly confirming the existence of a new game in the series. Both jobs are based in Kyoto and deal with level design in some fashion or another, suggesting that the project is in the early stages of development.
The first job, a 3DCG Designer, will revolve around “Topography” creation, level and concept design specifically for dungeons. The second job is more of a broad level designer, so they’ll be more generally on enemy and world design while also mucking in on the dungeons.
While the job listings don’t necessarily state that this is a new Legend of Zelda game, it seems unlikely that Nintendo would be hiring new staff members just to create DLC for Breath of the Wild, especially considering that it’s over a year and a half old now. Expect a new game in ther series, then, but don’t expect it for a while. Until then, you could always just playthrough Breath of the Wild again.