It's probably a Pokemon Sword & Shield rep. With the DLC coming soon, now would be the best time to reveal one. Personally, with the new Hyrule Warriors coming, it'd be nice to finally get Impa added. The Zelda series badly needs another rep with a unique moveset and I've been saying this since Smash 4, that Impa is the perfect fit. She's been a part of the series since the first game, and several of the games like Ocarina of Time, Skyward Sword and Hyrule Warriors have a younger version that's a fighter.
Plus the Zelda series hasn't gotten a unique new character since Melee. It's about time this series finally gets a new character added to Smash Bros again.