I frequently wish I had Ceylon's union + intersection types in Java. Typescript has them, but they aren't as rigorous. Also the collection classes were rethought and designed with immutability in mind (reading a list is a different interface from mutating a list).

Honestly, it was/is a pretty great language.

I think their biggest mistake was picking Eclipse as the IDE. They eventually produced an IntelliJ plugin but it was far too late. The serious JVM dev community had already picked IntelliJ. I don't know anyone who uses Eclipse anymore.