If you want to know how PostgreSQL compares to other databases, or are looking for ideas on areas to improve, the jOOQ blog looks like a good resource:
https://blog.jooq.org/. jOOQ is a java library that provides a database-agnostic way to access many different types of databases. I know it does some query optimizations but I don't know how advanced it is. The other reason for them to track advanced functionality available in some databases is to ensure that they support users who want to use those features.
Most of the blog is specific to java or its library but there's also frequent articles on advanced functionality (row values, window functions, etc.), query optimization, and comparisons between the databases. The comparisons of query optimizations in particular might be good low hanging fruit since it doesn't involve any user-facing changes, just recognizing a few additional patterns in the query parse tree and knowing a more efficient way to perform equivalent work.
Bear