No jokes: pick a language that is in the market, but has a different design philosophy than your background.
Your background includes compiled static, and loose scripting, with strong library tooling, so you have diversity there, so a language in which you have to think differently is the right choice.
I recommend:
Rust if you want something safe but that makes you work differently from java
Go if you want a real mental challenge (the coding approach is very different from java)
Lua if you want really see functional programming as a philosophy
Python if you want scripting, and are tired of the web
No jokes: pick a language that is in the market, but has a different design philosophy than your background. Your background includes compiled static, and loose scripting, with strong library tooling, so you have diversity there, so a language in which you have to think differently is the right choice.
I recommend: