The Nintendo Switch is receiving a new game in The Legend of Zelda series in 2019 in the form of a remake of Link's Awakening, but there may be a second Zelda title that has yet to be revealed and that may see a release later this year.
A prominent leaker named Vergeben has claimed (via Game Rant) that another 2D Zelda game is coming in 2019 that will be of a much smaller scale to that of the Link's Awakening remake and that it will be exclusive to the eShop. The rumored second Zelda title will also be released before the remake of Link's Awakening.
The rumored second Zelda title would most likely be a port of an existing game if it's coming out this year. One possibility is an updated port of Tri Force Heroes, with extra content and a better matchmaking system that uses the Nintendo Switch Online service. There is also the possibility of a remake of Four Swords Adventures with online multiplayer functionality.
It's good to see that Nintendo hasn't completely abandoned the concept of 2D Zelda games in the wake of Breath of the Wild. It's possible that the 2D Zeldas will be used as filler titles in the space between the development of the larger 3D games that will make up the bulk of the mainline games going forward.
If this rumored game is real, then it would make sense to announce it at E3 2019 for a release later in the year, as it's doubtful that we will see any more Nintendo Directs before then. We still don't know when Link's Awakening is due to be released (outside of "2019") which casts further doubts on the rumor, as the lack of a solid release date for Link's Awakening suggests it won't be released until the end of the year and Nintendo isn't likely to release two Zelda games that close to each other, unless the other game is a spinoff title and not a mainline entry.
The Legend of Zelda: Link's Awakening is due to be released for the Nintendo Switch in 2019.