For every single thing that you are able to build, you pick a language or tool that suits you the best and just keep improving. Each language has it's pros and cons but, IMO, what matters the most is the code quality, architecture and the things I mentioned above. Better focus on those skills.
If you write nice structured, good organized PHP code (e.q), that is stable in production and which is easy to maintain then you are far better engineer than someone who is showing off with his Rust or Haskell unstructured, not consistent and in the end unstable code. Don't let the trendsetters put pressure on you.
In the end, if you are are really bored with C#, try to learn some F#. It's also .NET but it's a different type of language and if nothing happens, in the end, you will write better C# :) Personally I think that SQL is also a good investment. Most of the people avoid it, I don't know why, but there are and there will be always jobs for a good SQL developer, especially in big companies where the salaries are also higher.
Good luck :)
For new interesting tech, Julia, NativeScript, and Rust look very promising. They will all have very different market appeal.
Julia will grow your type-systems, metaprogramming, and mathematics applications. Move you in the direction of scientific computing; It is used by several of the National Labs in the States.
Rust can help change the way you think from many other languages. can result in you becoming Concurrency and Security aware and friendly.
- Swift + iOS.
- Scala/Go + Backend development: distributed or real time systems.
- Python + Machine Learning.
Back end is pretty open. As others have mentioned Go is an up and comer. Hard to predict the market pull for it this early, but it seems promising.
Java is still pretty popular, (spring in particular) for web servers. It's not talked about much in the blogosphere but there's a lot of companies that still need java devs.
I can't speak to python/erlang, as I don't have experience with either. I think python might possibly be a better sell just because its popular in data science fields.
Personally I think c# is still pretty marketable. I wouldn't worry too much about the specific tech stack. These days stack-specific dev's are some a dozen, but finding someone who can pick up a new framework/stack/language quickly and efficiently are rare.
I've seen this too, but I haven't written Java since Java 7 lmao
So if you're looking at specific en-vogue techs that help with that, I'd say TypeScript is leading the charge right now. Apollo is also good, since Apollo shops will love you, people with any graphql will still see it as an asset, and basically everyone cares about web services in general.
If you're in a major metro area like NYC/SF/London etc then I'd suggest Java/Scala/C++. If you want remote then JS seems to have the most opportunities going.
In real I experiment a lot with my side projects. I develope android apps, build linux desktop apps in c++, make js micro-utilities,built a unity3d game, write wordpress plugins and list goes on. But when it comes to solid sel3ction I prefer matured tech, they are well documented; major bugs are already solved and edge cases are well defined by community.
Tldr; Use mature tech for main work. Experiment others on side