I've been thinking about the lack of ad-hoc SumTypes in Java. The sealed class approximation isn't good enough. I'm seriously considering taking up TypeScript in earnest as my 'go to' language. It's an outlier in supporting A | B | C and being mainstream with pretty good runtime performance.

One thing I'd have to check is the quality of IDE support for refactoring tooling. Refactoring operations for Java using IntelliJ almost always works and as I expect or better.