The next Super Smash Bros. Ultimate DLC fighters are Pyra and Mythra, the twin aegises from Xenoblade Chronicles 2.
Nintendo announced the two and showed them in action during today's Nintendo Direct, and we'll get to play them this March. Pyra and Mythra are technically one character since you can swap between the two at will, but they've both got their own play style. Avoiding spoilers, this is true to their relationship in Xenoblade, and it fits with the styles of other Smash characters like the Pokemon trainer.
Rex, the main character of Xenoblade Chronicles 2, is also making a small appearance through the ultimate moves of the two aegises, which are based on their level three artes from Xenoblade Chronicles 2. The rest of the gang can be seen in the special stage coming with Pyra and Mythra, which uses the titan Azurda as a base.
The aegis sisters are the star of Challenger Pack 9, the fourth DLC installment in Fighters Pass 2 for Super Smash Bros. Ultimate. Each Pack comes with a new character, a stage based on them, and some other goodies like music tracks and Mii Fighter outfits. You can buy individual Packs for $5.99, or you can get the whole pass for $29.99 up front.
Altogether, Fighters Pass 2 has introduced Min Min from ARMS, Steve and Alex from Minecraft, Sephiroth from Final Fantasy 7, and now Pyra and Mythra. There are still two Challenger Packs to go in Fighters Pass 2, and as Smash Bros. director Masahiro Sakurai confirmed last February, there are no current plans for additional DLC characters beyond them.