Another interesting project is to do your own web framework. Routing, ORM, validations that work both server and client side, rendering server and client side transparently are some of the challenges. Just do something and improve some actual pain point you see with a popular one. I did mine and, even thou I have never used it for anything important, I still think it is a good learning experience and I can point to some aspects of it that are better than all the major web frameworks.