No way could they make a proper sequel in just two years.Yeah holiday 2020 would be three and a half years using the same engine and mostly the same assets a sequel would be perfectly doable in that time. In fact I'd even say there's plenty of time for some tech improvements and new assets.
...checks google.
Yo wtf Botw released March 2017. Yeah this could be a holiday 2020 title. Wouldn't expect it to be, but it's not crazy like I first assumed.
Of course, we don't know what they're doing with it, if its an entirely new world just as big as BOTW with more dungeons and story its going to take a long time but if they're re-using the map not so much.
Personally not gonna get my hopes up for 2020, but I can't wait.