The music customization options in Ultimate took some good steps forward but also a step back, imo. Allowing songs from a franchise to be played on all of that franchises stages is solid, and of course being able to play EVERY song on BF and FD is very, very cool. However, removing the ability to play miscellaneous category songs on main franchise stages has taken away the cool theming and even references that some misc. songs had when paired with certain stages. Stuff like:
"Tunnel Scene - X" on Lylat Cruise (some of the X devs would go on to make StarFox, also just cool space themeing)
"The valedictory elegy" on Gaur Plain (Monolith Soft connection)
"Tomorrow's Passion" on Boxing Ring (which I'm almost certain is one of the only reasons the song was included in the first place)
And so on. Kinda lame, honestly.