One thing that helps me a lot when developing the level design is making a list of everything that can be done in a level and in the game.
Things like, how far the player can jump, maximum height and speed. All kinds of collectibles, power ups, enemies and mechanics.
Of course since you are developing the game you know 100% of what your game has to offer, but writing things down helps to keep everything fresh and avoid missing details.
Another good thing to do is play other games, but instead of playing for fun, try to see what kind of mechanics, enemies and power ups are common, and what would fit on your game. Of course don't only copy other games, improve everything you see and try to create your own rules.
Another important thing is, don't be afraid of things that looks hard to do. Of course some mechanics or AI are harder to develop than others, but we are here to help you right?
And then at least, with all your ideas fresh on your head, take a piece of paper a pencil and a eraser, and try to start to draw your stages on paper first.
The flow and vision that you have on paper is way bigger then on the computer screen. You don't have distractions, wrong layers, don't have to waste time scrolling up and down or sideways , you don't need to zoom in or out, adjust anything. The canvas is clear and empty, just let your ideas flow.
After that, you will try to copy the ideas from the paper to the computer and will end up changing everything, but at least you will have many more ideas on how to develop your stages ^^