Here's one that's most certainly unpopular, mostly because people care more about fighters in terms of representation than anything else.
Personally I think some series should have much more music and remixes to represent them than they have currently in Smash. For example, Donkey Kong as a franchise has just one song from Tropical Freeze, which has a really solid library of great tunes to pull from due to David Wise's return to the franchise, and yet most of what's there is just remixes of Jungle Hijinx alongside an admittedly good collection of DKC 1 and 2 songs. That's just one of the better ones honestly, Metroid couldn't be any less representative of anything other than Metroid 1, and even then it's mostly just a collection of remixes. The Metroid Prime Trilogy is a fan favorite and yet it in total gets 5 songs with one being the multiplayer theme which was a mode that doesn't really represent the game at all and another being a theme from the DS title? Like Prime 3, despite being filled with an assortment of memorable and very well composed battle themes gets absolutely zero love whatsoever whereas the pretty much universally hated Other M gets three songs and two of which being just boring "this is threatening" music. It's just so odd to me that franchises that aren't even Nintendo like Mega Man and Castlevania (which don't get me wrong they absolutely deserve the amount of music they have, I just wish they weren't the only ones with that much love put into them) get much more songs that are equally spread about through the series' lifespan, albeit leaning much more towards the classics.
To see what I mean, here's a collection of Metroid and DK songs that aren't in Smash that would work great as battle themes