How could they be able to do a Mainline Zelda, when they just started to seemingly hire over the summer? Hell, there were job listings for a 2D Zelda back in the Spring/Summer period of 2017.
I would imagine, they are developing the new Mainline, and it'd use a similar engine to BoTWs (but a bit more beefed up), but I don't expect that until Late 2020/2021 at the earliest.
If there's anything in Zelda next year, I'd place my chips on something from the 3DS Zelda team: Nintendo EAD.