It is quite logical after having opened the door with Mario or Pokémon. It was one thing to develop mobile games, and quite another to exploit their core franchises, so it comes as no surprise to hear that Nintendo is developing Legend of Zelda for mobile phones.
That's what the people of The Wall Street Journal tells us, so I think we can go a long way in giving credence to the story. The Zelda game would come after what we already know is currently under development: Animal Crossing.
The game is being developed between Nintendo and DeNA, and considering that Animal Crossing should appear in the mobile market in the second half of the year, Zelda can easily wait for 2018.
We have to remember that Nintendo's new CEO, Tatsumi Kimishima, said last month that the Kyoto company is embarking on production of at least three mobile games, until March 2018. Well, we already know the name of another one, We pay attention to the WSL, of course.
There are no details with which to soften the arrival, it is too soon even to speculate, but if there is a game with the high quality standards in Nintendo that is Zelda. The last episode of the saga, Breath of the Wild , has set the bar very high.