There have been a lot of changes to contributors, and so I'm not sure there's a clear picture of the codebase as it currently stands.

My suggestions:

• Analyse each file, determine if it is still required or should be removed
• Update where necessary for https://developer.wordpress.org/coding-standards/wordpress-coding-standards/
• Add other improvements (e.g. namespaces, type hinting, better code splitting)
• Ensure all files have adequate documentation (through docblocks, inline comments etc)