In my d20 Modern campaign, I used ley lines to explain magic hot spots that would be usable only at specific times, in specific locations.
Picture a single ley line as a ring that circles the globe and spins on a fixed axis at a set speed. Now imagine the world has many (dozens?) of these, which manifest at different strengths and spin at different speeds, each on its own axis. The intersections of these ley lines are the spots where magic gets a boost, and if you can chart the size, speed, and orientation of two or more lines, you can predict where areas of increased magical energy will appear.
To throw in another wrinkle that can help explain why the locations of these intersections aren't always easy to predict, the strength of many lines will ebb and flow, causing them to speed up or slow down accordingly. Also, if more than two lines intersect at the same location, they could become temporarily "stuck", stabilizing the area of increased magic for some amount of time and throwing off their travel in ways that are difficult to predict.