I'm designing a programming language, purely for fun, and want to add as many experimental features as I can, just to make programming in it something completely different, and that not in a bad way like Brainf*ck or Malbolge.

However, I seem to be quite bad at coming up with new things for it but I'm sure that there are tons of things out there that have been talked about but never really tried out.

• What experimental language features or concepts not implemented in mainstream languages are there at the moment?

E.g: If I asked this in, let's say, 1960, an answer could be "Object-oriented programming".

I'm sure that there are a lot of unimplemented ideas computer-scientists have (recently) come up with, at least I was told so.