When working with mashups, it's unclear what relationships should be used on the work level.
The music quotations are too long to use the "quote music" relationships, and when they are short enough, it doesn't convey the spirit that it's not really quoting, but actually integrating the original work.
Based on doesn't really convey the meaning that the original works are largely unmodified, nor if it's the lyrics or music getting mashed up

Having a dedicated mashup relationship would clarify how works were used to create a new one, and a relationship attribute could be used to specify whether the music is mashed up, or the lyrics, or both.

As an exemple, if we take this work: https://musicbrainz.org/work/b46e5b77-9e8d-4171-bcad-1adb9af1e20f
There would be 3 'Mashup music from' relationships (Omoriboy the pirate, Whiplash, Raise up your bat), and 2 "Mashup lyrics from' relationships (Omoriboy the pirate, Whiplash)

Another example would be https://musicbrainz.org/recording/54a4a9ee-251a-480e-8b74-daf4b26ffdf4 where it mashups a lot of recordings and works on top of Space invaders by MDK. While "quote music" could be used for The Flintstones theme, it doesn't carry the same meaning and weight as being mashed up into the song.

Discussion on the forums: https://community.metabrainz.org/t/adding-of-mashes-up-as-work-work-relationship/814625