What are some promissing FOSS projects?

I mean, universally promising. No niche shit. What are the next "Linux" and "GCC"?

Hopefully a structurally sound microkernel like L4, and a set of small but fast compilers for various languages (including haskell), written in haskell.

First though, we need to tackle the problem of the massively bloated machine code that ghc produces.