gap VS Mathics

Try implementing your own computer algebra system. Or look at the implementation of GAP (https://github.com/gap-system/gap)

There's Mathics, a subset of the language implemented in Python, but unfortunately after the main author of that was hired by Wolfram, the project seems to have basically died. Still fun for what it is.

https://mathics.org/

You could try to write more Mathematica functions into SymPy. In other words, contribute to the Mathics project (https://mathics.org/)

I'm one of the authors that spent years building SageMath; indeed, the goal of our project is to create a viable free open source alternative to Mathematica (etc.).

There's an interesting discussion of actual attempts to create open source implementations of the Wolfram Language in Wikipedia under "Implementations" (see https://en.wikipedia.org/wiki/Wolfram_Language). The first attempt was 30 years ago by Fateman and he received an official cease-and-desist from Wolfram for his attempts; he did a lot of work on open source Maxima as a result.

I think right now https://mathics.org/ is the most complete open source Wolfram Language implementation, and it uses Sympy extensively under the hood. Mathics was dead for a while when the main author got hired by Wolfram (see https://github.com/mathics/Mathics/graphs/contributors), but during the last year there has been an enormous amount of new work on Mathics.