Lack of skill results in changing routes. This is what you usually see on the upper routes: there are a lot of opportunities to go to a different route, literally by falling into it after missing some floating platform or other.

Sufficient skill results in changing routes. More available for lower routes: if you go into a quarterpipe with enough speed, or notice some subtle floating platforms, or something like that, you can find your way into somewhere a little easier, most basically by going up.

A simple choice. I can think of fewer examples of this off the top of my head, but there are cases where the level design can honestly give you a clear choice between two (or more) possible ways to go, with neither option requiring more skill to access at the onset. (One may take more skill than the other later on, but the choice itself can be fairly equal.)

Randomized. This happens in Chemical Plant Zone with some of the pipes, which take you in different directions depending on whether the second you enter them is even or odd. This isn't too common, and I can't say I really mind that.

I feel your discussion of routes emphasizes their distinctness at the cost of deemphasizing their interconnectedness. In GHZ1, for instance, the bottom (pink) route begins at the first pit, so it feels like it's an offshoot of the red route from the first pit. However, after the second pit, the pink route is descended from both the red route (circa the second pit) and the pink route (prior to the second pit). This brings up the matter ofbranching occurs, which is something you don't much touch on. I can think of a few fundamental possibilities:It's also tempting to combine the first two possibilities into one, specifically "one route for those with enough skill, another route for other people." This, though, arguably comes at the cost of losing the description of routes as wholes, as you do in your images: instead a route would only last the interval between one branch and the next. The high/low/middle route distinction would be replaced with a high/low/middledistinction: some areas are easier, some areas are harder, and different routes snake in and out of those different areas and may fork off into other routes in the same areas or else in different ones.