There's a Nintendo Direct presentation airing tonight at approximately 10pm, but it seems at least one announcement has leaked early. Online retailer Rakuten uploaded a series of new images for Mario Tennis Aces on Switch, and updated the game with a Japanese release date, which would appear to be based on information from tonight's Direct.
The page, which has since been removed, listed the game for a June 22nd release in Japan. Due to the Switch being region free, firstparty titles generally release the same day in the UK as they do in Japan.
The store page images confirmed Mario, Wario, Waluigi, Daisy, Rosalina, Yoshi, Donkey Kong and Bowser as playable characters, as well as the store text confirming motion control for tennis play, local multiplayer, and online multiplayer.
While this isn't an official announcement, the presence of new game images in the short lived store page heavily suggests the information is accurate.