- Test::Mojo::* needs a better API.

- Chunked transfer encoding could use better abstraction, it's currently
  split between Mojo::Message and Mojo::Content.
  
- The ability to add more content handlers besides Mojo::Content and
  Mojo::Content::MultiPart might be interesting.

- A lot of code needs to be cleaned, especially around Mojo::Message.

- Tons of RFC spec tests need to be written.

- Documentation.