Here is a roadmap that we can follow:

• Finish runtime library like comptime_eval.h in LFortran
• Be able to call a function like lpython_str_format that is our own LPython implementation of the % formatting operator, this function will be inserted by the frontend (tested by Implement real and imag for complex #256, which calls _lpython_imag)
• Implement the rest of str functionality (mostly in the LLVM backend) and fix all bugs related to strings (the remaining bug seems to be Fix string indexing #273)
• C interop (Implement calling into C #3)
• Generic functions (Figure out how to handle generic functions #141)
• NumPy support (How to declare a local array #5)

This should allow us to implement large parts of the runtime library, and get LPython off the ground. After the above works, the other missing features that we need to implement are:

• Generics (Implement generics #154)
• Interactivity (Interactivity #329)

Other (lower priority):

• Improvements to C interop: Roadmap for the C backend user experience #545