- Levels where you can't see everything because it's dimly lit. One of desgeeas levels gave me the idea for it (can't remember which) and I used it in Tower of Joy, and Talps even did it a little bit in I dreamed of the Sky. Basically you use the fire or some other decoration and only show a few blocks. Then you use the invisible block and a lot of black spaces to create some required extrapolation to where the jump platforms are.
- Several times in my upcoming level, I've found any jump where you have to switch directions in midair, like jumping vertically up a series outcropped stones is usually difficult enough to at least keep the player entertained.
- In Tower of Joy, I had a part where the fire spitting fire decorations shot at the player. I also used the sticky tile such that the player had to jump to even move, making it difficult to avoid them. I felt this was Very Hard, and had trouble with it even on my own playthroughs. (Now that I think about it, Run and High Jump are the only powers I used in Tower of Joy (until the secret part). It's a quick run through if you want some more ideas.)
- Another one I used involved strategically placed hazards (I made the fire decoration a hazard by overlaying a small invisible red hazard) while the player had to jump up and around a series of 1x1 blocks. Not particularly difficult but also not particularly common and forces the player to pay extra attention.
- There are other things you can do to enchance suspense without making the jumps any more difficult. For instance, making the entire ground hazardous. Adding spikes to the edges of blocks. Adding hazards in the middle of jumps that they have to make sure they jump over (even if they would have already). I liked a section where I made the ground hazardous lava and put it three screens down to enhance the idea that the player was super high up to add to the feeling of suspense.
- You can always use lasers or the moving spiked blocks to force the player to time their jumps. There's also the robot that dropps sparkling bits every second or so.
- The robots that chase the player (slow, medium, and fast) can be used to add a sense of urgency. This is done well in Don't Stop Running, Monocromatic Grotta, and many other levels.
EDIT: Wait, you're Healy?? Asking this question? Psh, you know all this already!!! You're an expert at making levels. What do I know compared to you?? X)