Mark Morgan’s music was not that homogeneous. There is some variety in his soundtracks, but I wouldn’t say that any of his songs would be inappropriate for a Fallout title. They all fit pretty well.
It would be tiresome if they did use the exact same songs from the classic entries (Fallout 2 did this somewhat, but it also introduced new tracks), but they don’t need to depart from the tones and moods of his tracks. Genres such as rap, rave, renaissance, heavy metal, or whatever, are fine on their own, but they don’t belong in this series, and they shouldn’t be inserted simply for the sake of adding new content.