We know they're from ARMS, and ARMS gets a free trial to celebrate
http://www.nintendoworldreport.com/news/53300/first-character-from-smash-bros-challenger-pack-2-reveal-is-a-stretch
This June will bring the announcement of a new DLC fighter to Super Smash Bros. Ultimate, and it's going to be a character from first-party fighting game ARMS. Today's Nintendo Direct presentation revealed that the new character will be someone from ARMS, but no further information was provided. At the very least, we now have a little bit more detail to narrow down all those guesses about who's getting the next invitations, and we'll only have to wait a few months for final confirmation.
Also announced today was a free trial of ARMS. It's exclusive to Nintendo Switch Online members and available to play between March 26 (9:00am PT) and April 6th (11:59pm PT). If you haven't tried ARMS yet, now is a great time to see what all the stretching is about, and the free trial is for the full game.