I can't help you myself, but I can share what I've learned.
My views on level design changed SO radically after I got my character physics in Unity working better. Excepting the case of high-traction characters like Mighty, my Sonic now has very, very predictable limits. I can now calculate exactly how tall a wall Sonic can run up before he loses all momentum; even turning around to run back down will not build enough momentum to overcome this limit. If the player does try this, the speed loss will always halt Sonic at the same height.
This predictability makes it much easier to design ramps, pipes, obstacles and such. If you can understand the maths behind SonicGDK's physics and make such predictions, you can design better levels. Start by a vague concept of scene set pieces, merely in terms of their general geometry. Spagonia's clock tower and city wall, for example. Then think of how you want the character to interact with those larger set pieces. Do you want Sonic to run up the side, use it like a tunnel or building, or perhaps travel around it? Perhaps multiple options? From there, think of set pieces which make sense that could provide this functionality.
Like Spagonia's rooftops and streets, leading from interesting setpiece to setpiece, with those pieces being interesting to traverse in themselves. If you have predictability, you can know exactly how to place the parts that lead between these zones.