Personally, the language that’s taught me the most to learn has been Haskell. It has a lot of very interesting ideas and a learning curve that plateaus after most other languages. There are several ideas that have trickled down from Haskell to other parts of the programming world and learning about them in the context Haskell is in my opinion better because you’ll learn about them in a context where they fit in with the rest of the language very well instead of being late additions that offer an alternate way of doing things.

Coming from Java and JS, Haskell has a very different approach to a lot of things so you’ll have to re-learn a lot before you get productive in it. This can be frustrating for some but you’ll learn more if you get over that hump on the other hand.

Haskell doesn’t see very much industry use and arguably isn’t very well suited for industrial application (I haven’t used it professionally so I don’t know personally) so it might not directly help you land any new jobs but it is in my opinion it’s a very good way to develop as a programmer.

Id suggest rust, gets you a step closer to the hardware and a bit of a different paradigm than Java while still feeling high level.

Python seems to be a staple for as long as I can remember and it looks like it’s still gonna be going strong for a good while yet!

I’m thinking of taking the dive and finally learning it myself soon.

I can recommend Rust - I’m training a few people on it.

And a pure functional programming language like Haskell or Scheme, if you don’t know what functional programming is, or are not comfortable with it. Functional programming needs a different mental approach to traditional (imperative) programming paradigm. Some of the more modern languages like Rust, JS and Python incorporate a lot of functional programming constructs. So it makes sense to learn them.

And a lisp - Common Lisp’s popularity is a public secret. Scheme is also fine. This family is homoiconic (program and data are treated more less the same). The syntax is actually very close to its AST. This gives Lisp unparalleled metaprogramming capabilities - mostly through macros. Macros in traditional languages are nowhere near Lisp Macros.

If it interests you, study a stack based language like Forth or Factor. Though they feel very different from Lisps, they have similar underlying properties. And you get more or less the same advantages.

I recently picked up Rust, still a beginner, but it’s very nice. And the compiler errors and tips are top. Since I used only Python before, Rust is my first compiled language and I’m glad it was Rust. Options and Result’s are also something I appreciate tbh. Other than for AI, simple stuff and maybe prototypes, I would use Rust over Python. Definitely recommend it. The borrow checker is a pain in the ass though. I’ll also be using it later for embedded systems when I become more fluent with it. I am also currently making a big project with Rust that I haven’t been able to do with Python.

Rust

Download GODOT and learn the scripting language and the shader language. Make a game! Or at least neat shader art.

Bonus answer: Csound. Make funky noises with code.

I have a couple of suggestions based on what you want!

Ocaml: A good intro into functional programming

The languages you’ve used seem to be primarily imperative style. Ocaml is not commonly used in the industry (with some exceptions), but it has a vibrant small community, and more importantly, because it’s a functional language, it will be different enough from what you know. Even if you don’t use it in industry, it will teach you a lot that would be hard to learn using non-functional languages.

Ocaml is a compiled language, with a very fast compiler and very good type system.

Scala: the only semi-mainstream language that blends OOP and proper functional programming well.

Although scala is mostly a functional language, it still has most of the gestures of imperative OOP languages like Java. It is also far more mainstream than ocaml and has used in industry.

Scala also runs in JVM like Java, and thus gives you access to most JVM libraries.

Rust

Intro into low level programming Other answers spoke enough about Rust so I won’t dive here

Zig

similar to Rust in low-levelness, but less mainstream, better C interop, and other interesting differences

Raku: the result of 15 years of programming language design

Raku, formerly perl 6, was almost redesigned from the ground up to resolve complaints of perl 5. It has a very interesting design, with a gradual type system, a blend of functional and OOP paradigms, plus interesting syntax features like sigils and grammars.

There are many interesting languages out there. I could talk forever about this, but I’ll leave you with the above. I personally would recommend ocaml if you’re interested in functional programming or care about performance. Otherwise, Raku, since its very interesting but doesn’t perform well (Raku’s VM is still immature and under development).

C# is a great choice.

Incredibly versatile language and should be an easy jump from java.

I’ll suggest Elixir. It’s a language that runs on the same virtual machine as Erlang, which has proven to be great for ultra-reliable and excellent at managing many, MANY concurrent processes.

Elixir itself builds upon this great foundation with a syntax similar to Ruby, but entirely functional. It’s a delightful language to read and write.

Rust, haskell, python, c++ are all interesting choices. I would argue that c# is too close to what you already know to be interesting.

If I were you though, I’d pick a project first, then decide what language makes sense for it.

While it’s related to your job and not very new compared to your current stack, it’s very worth it to learn typescript. It has a cool type system and makes frontend development sane.

DotNet is closest to Java, but hang on to your hat: the state of C# is at least half a decade ahead of Java, if not a full decade. It’s sophistication will make Java use feel like banging rocks together. DotNet Core can now run on all three primary platforms, and with some careful work, you can write a single program that can compile down to each platform and carry along its own required binaries, no pre-install of any framework needed.

My second recommendation would be Rust. Stupidly steep learning curve, but an absolutely game-changing one where safety and security is concerned. It’s my next objective, personally speaking.

Any other language I could recommend starts getting into speciality purposes, which makes general use more difficult or even wholly inappropriate.

For example, if you are dropping into DotNet for business applications, I would also recommend diving into F# for that functional goodness for building complex business rules and data handling. But building an entire app in F# can be jamming a round peg into a square hole under many circumstances, it’s appropriateness envelope does not cover as many cases as C# does. You want to use C# for boilerplate/frameworks, F# for the core bits where it is going to shine.

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:

1. Rust if you want something safe but that makes you work differently from java
2. Go if you want a real mental challenge (the coding approach is very different from java)
3. Lua if you want really see functional programming as a philosophy
4. Python if you want scripting, and are tired of the web

C. Plain old low-level “portable assembly” C.

In particular I’d recommend a Game Boy game, because GBDK uses SDCC and is fairly low-bullshit. With scanline interrupts, you have a surprising amount of time to do clever visual effects, flexing the machines one-and-a-half background layers. The machine is sufficiently explained in about forty-five minutes via the Ultimate Game Boy Talk.

(NES is also an option, but - don’t. I say this with my NES homebrew project open in the other monitor. It’s a miracle this thing ever caught on. You need a decent understanding of the hardware, C, and 6502 assembly, just to put stuff onscreen efficiently. And then the fillrate still blows. The Game Boy can blast the whole tilemap in a fraction of a frame if you cheat hard enough.)

If you want more power, the GBA supports C and C++, and that little fucker runs Doom. It’s intended as a super-flexible sprites-and-tilemaps affair, like an SNES with an upgraded graphics chip. (And a downgraded sound chip.) You can scale and rotate dozens of sprites at 60 Hz, atop two scaled-and-rotated backgrounds. Or use fullscreen bitmaps in 8-bit or 16-bit color. (Well, 15-bit.) The homebrew scene for the GBA started before the console even launched, because it has a pretty standard ARM chip and will do anything the Genesis could without breaking a sweat. There was nearly a voxel heightmap version of Banjo Pilot that looked pretty damn close to Diddy Kong Racing on N64.

I recommend C and retro consoles because they’re suited to one another - and they offer control. You are in command of every single byte on these machines. C is the ideal language for that. It offers familiar high-level structures. It supports familiar syntax. But it runs very close to the metal, and if you want to get even closer, it will let you.