[psytrx]

For me, there is no single go-to project that fits all languages. Each language has it's own purpose.

Go and Rust are fairly similar in terms of parallelization and performance. Both are general purpose languages and are great at solving system problems. Choosing a toy project that requires these perks will yield the most satisfying results and enables you to get a good grasp of why those languages exist, and where their differences are.

Both languages empower you to build inter-system applications, both languages are highly performant and both let you build safe and tested programs. A distributed ray tracing engine comes to my mind - it would allow you to let both languages collaborate, compare solutions to problems between the languages and get into the nitty gritty details of memory management, allocations, garbage collection etc.

This would probably be my pick, because you can build and improve it for years if you really want to put the effort in it.

Here's a great primer on that topic - it's still actively developed and made me look at modern ray tracing technology in awe.

https://raytracing.github.io/