Parsing but also casting to string efficiently is still an open problem with regular breakthrough. Ryu or Dragonbox for float to string come to mind.

Parsing with errors that are useful for the human is definitely an open research topic.

Capabilities have seen interesting work, in particular in relationships to effect handlers.

Hyperloglog and other datasketch keep seeing incremental progress. I have not had time to read it all but Omar Ertl has a massive reduction in memory use in UltraLogLog that i expect a paper on soon. The code is already up.