This track, Crimson Canyon, came together surprisingly quickly -- mostly because it's just a bunch of landscape stuff though. I could see myself coming back to these tracks later to make them better, but I'm pretty happy with the three I have for Prehistoric Park (one of the worlds in the game).
I have one track for another world that has a medieval / dragons theme, which slots into the Fantasy Fields world (I'm trying to make everything alliterative), which leaves two more in that genre.
I do want a haunted mansion / ghosts kind of track, so I'm leaning toward starting that next so I'll have two in the next world done. But I expect it will take longer given it will require more structures, props, etc.
After that, a third world I want to do is "Tech Town", for all the sci-fi techy themed tracks I have bouncing around in my head. But that's a bit of a ways off.