I've worked on projects where the music adds different parts based on in game decisions. I've also done a lot of "spacial" and ambiance things. An example of this would be, maybe you're standing right outside the town so what you hear is that town's music really faintly and panned over into the "distance" while you're outside, but it goes to the center and returns to full volume once you enter.
I also really like turning major motifs into music effects that can be played at vital times, I usually break silence with these before moving into a song. I consider silence an equally powerful tool during the right scene. I really could just sit here and talk. Because then I just have ideas on top of ideas.
What I really wish for is a proper way to crossfade into different tracks. There is so much I would do if that were a possibility! Like how Grant Kirkhope's music would change to that exact point if you walked into a caved area or dove underwater. I'd be so about that.