At the root of each level sits an idea, a concept that you wish to explore. I've found it historically very difficult to work on levels I wasn't convinced by. If I couldn't even convince myself that the level was good, I would power through it or abandon it. When powering through it and pushing it out as a rushed product, other people were also having a hard time being convinced by it, to no big surprise. You and your testers are basically a trial run for your levels, but where to even begin?
There are two points where a level technically begins. Once, when you come up with the concept, and once more when the player starts playing. I'll focus on the former first, because it makes designing the latter much easier.
The Designer's Start
Simple question: "Why are you making this level?"
There are a few kinds of answers to this question, and from the answers it's easy to discern when you might be bottlenecked by your creative ideas for the level. I've found that the best answers to this question come from passion, not filling slots in a world list. "I wanted to make something with Dolphins", rather than "I needed a pyramid level for my desert world". To clarify, it's absolutely possible to give a passionate answer to a level that is intended to fill an episode slot, by alluding to the core idea that takes place within the pyramid: "I wanted to make a level where Toad goes treasure hunting and the player must evade Indiana Jones-style pyramid traps!" Now the ideas come sprawling.
This question is incredibly useful for 2 reasons: First, it makes it easy to explain your level when necessary. But more importantly, the answer provides a common thread for yourself while designing. When making such a pyramid level, you might find your thoughts immedaitely shift to "okay, what kind of traps can I come up with. Giant boulder? Trap door? Fake treasure?" It gets the creative juice going immediately and designing many sections of the level will be much easier because of it. Compared to "I wanna make a pyramid level" where you might've just taken a pyramid tileset and put down some blocks and enemies, because that technically is enough to fill the pyramid slot in the episode, even if the tileset and music could be replaced at any point and nobody would notice.
But once you have a concept on your hands which you are intrigued by, that's a very powerful shortcut to get going and have fun designing.
Try to use this on your own levels and take time to conceptualize more ideas before starting to design if you feel like you might lack the means to properly finish your level! If you have fun making the level, chances are you and other people will like it for months and years to come.
The Player's Start
Now that you have designed your pyramid level, how does the level start? This section is very reminiscent of a lot of youtube video essays, which are stealing my job!!! (jk)
Essentially, the first obstacle the player should encounter should do a good job at getting the player up to speed with the next obstacles they encounter. For this Pyramid, Mario might try to open a door, but instead, a trap door opens below him, dropping him into an ancient prison with a broken door. It sets up the setting, and indicates that things are not always how they seem. If you then also rob them of their first powerup by making it run over a pressure plate that causes a crusher to crush it, you taught players to be aware of pressure plates without robbing them of a life, too. Make sure to actually give them a powerup later, though, for all the non-instakill threats. Unless there are none, in which case, keep them on their toes, and make sure to tease with checkpoints that are barely out of reach ;p
That was a fairly lengthy description and I kinda veered off because I got too excited, oops! To boil it down, look at what kind of obstacles you provide and make a harmless version of them at the start. A screen that makes players go "ah, got it! so that's what we're doing" doesn't just make for a good pickup moment to get players going, but it also works as good screenshot material for episode threads ;)
I hope this was helpful!