During the September 2019 Nintendo Direct, it was revealed that you will be able to get a Mii Gunner costume of Sans from Undertale. It will even come with a special music arrangement of Megalovania that is composed by Toby Fox.
Yes. It's true. You'll be able to buy a Mii Gunner costume of Sans in Smash Brothers Ultimate later today! It'll come with a special arrangement of Megalovania composed by me. It's a huge honor to have something I created in Smash. Thank you so much Mr. Sakurai!!! pic.twitter.com/eKDhwSKOJj— tobyfox (@tobyfox) September 4, 2019