If you still want to contribute to tools like Swift Argument Parser or Swift DocC outside of GSoC I'd be happy to help you to find interesting issues to work on and/or discuss ideas you might have for enhancements and guide you through the process of pitching such an enhancement here in the forums.
That said, similarly to what's asked in Can I participate on gsoc projects as a non-gsoc contributor?, if there's overlap between the improvements you want to work on and an accepted GSoC project, we should want to support that GSoC student and do what we can to make their project successful.