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Genre Mashup- The expectation with musical collaborations is that while you're getting a mixture of the two styles, they frequently lean more toward one than the other. "Breakn' a Sweat" by Skrillex and The Doors is decidedly in Hard EDM territory, but it's also heavily colored by the style of the Doors in a way that isn't just shown by the Studio Chatter Skrillex sampled into the final mix.
If you've got an especially off-the-wall collaboration, you can hit Weird Crossover territory, and the resulting fusion can be a genre oddball for everyone involved. David Bowie and Bing Crosby's "The Little Drummer Boy (Peace On Earth)" collaboration is a solid example listed already, and the combo of crooner and New Wave ends up sounding a bit like jazz-rock in the vein of Chicago (Band).
A Wild Rapper Appears! is when the collaboration takes the form of one of the artists "hijacking" the song into a different genre for a stretch, the trope naming example being rappers dropping into pop songs for a while.
Edited by Scorpion451
A Special Guest has an influence into changing the genre of a work into what they are known for.
Ex: "Oops, now that you invited Elton John to play on your Speed Metal song, it now sounds like a slow piano ballad."