What is missing?

There are lots of ways in which Mathics3 could still be improved. FUTURE.rst has the current roadmap.

However, many WMA Built-in functions have not been implemented. To get a list run:

git grep -n "# TODO: " mathics/builtin

from the Mathics3 core github repository.

We have a tutorial for adding new Mathics3 functions.

Many of the existing Mathics3 Built-in functions are incomplete or work differently, such as checking for different error conditions.

Although we can always use help in Python programming, there are other ways to help. For example:

What does Mathics3 offer?
Why recreate Wolfram Language?