I think they'll do a 2D Zelda for the Switch now, a 2-player Zelda I'm guessing (one player plays Link, the other Zelda) that you can play with just one Joycon, a little bit like 4 Swords but with two people. And that'll probably be 2019-ish. 

For another 3D Zelda ... I dunno ... I wouldn't entirely be surprised if this is the only new 3D Zelda the current Switch gets.

Assuming a development time of 4-5 years for the next game, it may be an early Switch 2 or "New Switch" title. The current Switch has problems running BoTW as is, maybe the next Zelda being on better hardware may not be such a bad thing.