In the low-level front, I've used Assembly, C, C++, and willing to learn Rust (or D?).
When I was freelancing, I was willing to learn Elm in case I needed to do some web+frontend development and they didn't let me write TypeScript in strict mode (plus tslint).
Or Ceylon/Kotlin/Frege (along with ParparVM if iOS is needed) or Swift, in case I needed to implement mobile apps and they didn't let me use Xamarin.Forms+F#.
In regards to databases, I'm an old-school guy who thinks normalized data usually beats any NoSQL/NewSQL solution, except for rare BigData indexing/denormalization scenarios.
If you're interested in F#/Rust/crypto remote job opportunities, ping me.