WARNING: Long-ass post incoming
Besides DK and Xenoblade I truly feel the music selection for Metroid could be significantly better. Like, multiple Brinstar themes/remixes, they didn't add the classic Norfair song which originated in Super Metroid until Ultimate, the ****ing multiplayer theme of all the tracks Metroid Prime 2 has, no Zero Mission music (granted, it's a remake of the original Metroid, but Samus Returns has 3 songs so I don't think that's a problem?)... I was also gonna mention how there are 4 vs. Ridley tracks but I guess it has a pass.
Only 2 songs from Super Metroid, a lot of the music comes from the first Metroid, the GBA games have the astounding amount of... 1 track, the Prime original triloy selection is also very questionable, when not counting remixes those games probably have the fittiest Metroid songs for a fighting game such as Smash.
Granted, for the nature of the series most of the songs additions would probably require a remix. For example Fusion has some great melodic ambient songs that could have pretty cool remixes, like this one
Nightmare Boss Battle Theme should be in as well, very iconic boss in the series
Serris battle theme could work perfectly for Smash
Moving on to the Prime series, you have songs that already work fine for Ultimate like
Popular and great songs like Phendrana Drifts, (which by the way is one of the best tracks in the series) which would totally deserve a remix
Another iconic track that could totally be in Smash
Torvus Bog theme from Prime 2
Quadraxis Battle song could fit very well
Not gonna forget this battle theme, which honestly, fits super well into Smash. It's pretty epic and barely ambient, which is strange for Metroid games, so there's no excuse
Bryyo Theme from Prime 3 has potential as well for a good remix, in my opinion
Then we reach to the biggest omission, one I'll truly never understand. NO Dark Samus battle theme when she's a playable character. I'd understand it if her battle songs were ambient and chill AF, but that's not the case at all. They even had 2 versions to choose
Prime 2
Prime 3
I know for some people this post will seem like excessive complaining or being very ungrateful, but I can assure you that's not what I want. It's not like I despise the music selection or something, just that it feels very, VERY safe to the point I find it boring. Like, Metroid is my fave Nintendo franchise and I barely play on Metroid stages because the music selection for the series feels pretty bland. Similar case for Donkey Kong which also has the 'problem' of "hey it's DK so I guess absolutely ALL the levels and locations in DK games are jungles and whatnot, right?" Xenoblade I guess has a pass in terms of music because in comparison it's a young franchise, but still...
I don't know, sometimes I feel like if certain franchises were added in Ultimate they would have received a better treatment overall.