I always think of what would be popular in the future. I would stay it will be an opposite reaction to the music we have now. Musically, mainstream music has changed so much. EDM Electronic/dance music is popular (though its popularity is fading). Right now its more about the artist, less about the record companies. People right now want to know everything about their favorite artists, its more interactive and social now.
I am going to say this now: Pop, rap, "alternative," "rock," and country are still gonna be popular genres. Especially in the mainstream
Even so, I feel popular music for teens in 10 years will probably be more conservative than what we have now (lady gaga who? Miley Cyrus who?). It will probably be calmer, less overtly sexual, crass, and /vapid (there will always be sex in music, but not as vulgar as now), more spiritual music, probably electronic voices (not real human singers or human singers singing/playing music alongside robots). It will feel more natural (even if its manufactured mainstream bullcrap!!!). That's my opinion. What do you think?