IMO the quality of arrangements from Smash 4 to Ultimate has gone up a lot, at least from what we've heard so far. The only ones on the website and in character trailers I dislike are Gear Up For..., Id (Purpose), and Bomb Rush Blush.
The option to have any song from a series on any stage from that series is basically all I ever wanted. There are so many songs I wish were on different stages.
For the music itself, I'm mainly hyped for the new DK songs. I love Tropical Freeze's OST and I hope we get a bunch of tracks from there, and the remixes of Gang Plank Galleon and Bonus Room Blitz are amazing.
Kirby has 39 songs in Ultimate if i'm not mistaken, that is about 8 more from Smash 4. I really hope we get Under My Control and the org Zero Two.
I'm hoping that Pikmin has a good selection of music this time, i'm not a fan of the two new arrangements they've shown (Main theme is meh and I never liked Garden of Hope in the first place.) Ai No Uta and Tane No Uta aren't needed, but I would like it if they came back. I'm hoping for Wistful Wild, Titan Dweevil, Pikmin 2 Boss Theme, Cave Medley, Plasm Wraith battle, The Forest Navel, Pikmin credits and Namida Ga Afureta.