For about 25 years there's been a small but dedicated group of people making romhacks for Super Mario World, and the early stuff was pretty quaint -- mostly rearranging tiles and enemy placements to make new levels. But over the years people have done some incredible hacks with custom graphics, enemies, mechanics, you name it. I've played some of them on real Super Nintendo hardware with a Super Everdrive which allows you to load rom files to the cartridge memory from an SD card. It's amazing what some people have made.
It's easy for people to think "Why would you constrain yourself to a DOOM engine when you could use Unity/Godot/Unreal/whatever and have better graphics and more freedom?!", but that's kind of missing the point. The point is figuring out how far you can stretch something, and how to get the most out of something limited.
Blade of Agony, for example, exclusively runs on GZDoom, which while more advanced than the vanilla DOOM engine, is still a lot more limited than Unreal or something, but I think that what they did to really stretch that engine to its fullest extent has made something that's extremely fun, charming, and gives it a distinct look and feel that you simply wouldn't have if you used a "modern" engine.
My level design skills are pretty weak. Being able to partially compensate for that by getting built-in nice character movement, enemy design, etc for free was really really nice. If I wanted to do more 3D stuff right now I'd almost certainly be using something like Mario Builder (I think stuff like Doom custom maps get at a similar idea).
Obviously I could have as much flexibility as I want if I learned Vulkan and wrote all the graphics calls myself, but of course that's really hard and time consuming. Pico-8 being restricted makes it easy for me to quickly hack something together just for fun.