OH MAN do I love this idea! I love the idea of building up your own platformer, I love the idea of spawning in a random preset element, it's such a fun concept, and I really liked the platformer elements that you included! A very nice set of distinct elements that really showed off the concept! Way to go!
I only wish it didn't end so soon! I would've played an unlimited time version for longer than the allotted time. I think the concept could be really well served if the challenge of the game came from more of the platforming than having a time limit. Especially because the game is so fun, I didn't like it cutting me off so suddenly. One idea might be to have a set of elements of increasing difficulty, and you only add them to the mix or the probability that they show up increases with your height, and you're trying to climb as high as you can. Then you can have a "Getting Over It"-style danger of falling all the way down and losing progress, but you can keep building your own course for as long as you're having fun.
I really enjoyed the idea, and I mean it when I say my biggest critique is that it kicked me out too soon. And while this demo is small and a little short, I think it demonstrates the core concept really well, so playing it makes me imagine what the fully fleshed out version of the game would be. Nice job!