There's a game out there IIRC, but from experience, it's easy to get lost in your own code because of the obscurity of bf. That's it's charm ahahaha.

There is a brainfuck variant supporting syscalls. Seems like a great candidate for a llvm backend. You could have mostly any app you want.

There's a few but they're usually incredibly simple like a calculator. It's mostly just a fun way to challenge yourself