The Donkey Kong Adventure expansion is out and while it's not on the same level as the Splatoon 2 Octo Expansion, it's still quite fun and well done.
The DK Adventure is only available after you beat chapter one of the main story and features only three characters: Donkey Kong, Rabbid Cranky Kong, and Rabbid Peach. Sadly the two new characters can't be used in the main campaign. Rabbid Peach starts with preset abilities and whatever you acquire won't transfer to the main campaign. With the exception of shared characters the DKA expansion is effectively self-contained.

DK and Rabbid Cranky are fun characters. DK can grab most enemies and even half destructible covers and throw them around, swing on vines to traverse stages, and is armed with a boomerang banana able to hit multiple enemies in one action.
Cranky is armed with a shotgun, can put enemies to sleep, and has a powerful jump attack.

As with main game missions and stages put character abilities to the test, though I've found the whole expansion a bit on the easy side, although even in the main game the difficulty laid in completing stages within the turn limit for the best ranking rather than clearing them altogether. Puzzles break the monotony between missions, though I would have preferred a few more combat missions and a bit less puzzles, which are almost entirely based on placing sliding boxes over switches.
The expansion is as long as a chapter in the main game, more or less, with exclusive challenges, weapons, and collectibles.

Oh, I think there's a bug in the final mission that prevents the background music from playing, but not sound effects or short musical transitions. Weird.