I'm really wondering when they'll have time for a release... Anything before summer is way too soon with them having absolutely nothing revealed except that its supposedly coming this year. June (or July? Can't remember) has Fire Emblem which is a huge Nintendo title and they won't want anything competing against each other. Then you have Pokemon coming in late 2019 which is, to my guess, October or November looking at previous releases. So this leaves us with August or September. My gut is (unfortunately) telling me September.
Either that or they're going to delay it, as much as I hate to say it. They knew fans would be angry if they put Isabelle in Smash and had nothing about the main-line series to announce. I think they threw it out there to keep an outrage at bay and they might not be able to follow through with it. I hope I'm wrong though and we see something about it soon, but its March and at this point I really don't think we'll see any game footage until E3. Nintendo is welcome to prove me wrong though.