Inspired by both default tileset 176 and Super Mario World's overworld. I might eventually add a second tileset for things like castles, rocks, hills, and other stuff.
To use the two-tile high path, make the bottom one solid and the top one not.
To use the one-tile high one, use Knytt Stories +'s object 16:25. You might be able to do something with KSEx, but I haven't used it.
An example of what you can do in Knytt Stories + with this tileset. The shifts lead to a one-screen "level" like the warp pipes in Super Mario Bros. 3. I also turned on the overlay, making layer 2 objects appear over Juni, so Juni actually walks under the trees as she goes by. Unfortunately, that makes it harder to fill in all the corners. On the level to the right of the bridge, the ground is on layer 0, and the tree is on layer 1 (so Juni won't go under the trunk), so if I want to fill in that corner spot, I have to put the corner on layer 2, having it go over Juni's face. You could always turn off the overlay (or use default Knytt Stories) to avoid that problem, but you'd have to move the trees out of Juni's face.