Scheme is big and small at the same time. It is old, but the last report on the language was released a couple of years back and it is still growing. There is still many areas that needs some love.

Checkout http://scheme-lang.com/

You beat me to it!

I'd recommend checking out Chicken Scheme. Not only is it a kick-butt implementation but its community, #chicken on Freenode, is truly fantastic.

Give it a whirl :)

I'd try Racket (Scheme family) or ReasonML.

there are plenty of older but less popular languages that could use some help to improve the ecosystem.

pharo smalltalk for example. (i can give you a list of things i'd like to have there right now.)

pike is a nice language with a small community. plenty of room for interesting things to add.

red-lang is another one that could use some help.

bonus: if you pick that up any of these three i'll offer to mentor you :-)

https://www.idris-lang.org the book is great.

https://urbit.org/docs/learn/hoon/ has a small, welcoming community. It's pretty out there though.

There's also nim (fast Python) and Crystal (fast Ruby).

I've been meaning to check out urbit thoroughly for a while but they seem to have gone overboard with the esoteric terminology, at least for me personally. I was put off by the naming schemes in the docs last I looked at them about a year ago. I suppose I'll have another look again. Maybe they grow on you once you are familiar with them.

Odin[0] is very new and has a pretty small community, but it's promising and very well thought-out.

[0]: https://github.com/odin-lang/Odin

I'd recommend Dart. It's a great language but the ecosystem was severely lacking when I lasted used it.

An additional benefit is that it's super easy to contribute to the Flutter codebase!

What do you consider as "run-of-the-mill" libraries ? like data-structures/algos ? or say something like a database connector / http router ?

Nim sounds like a perfect opportunity for you :)

Zig is very cool. Not sure whether it has a community yet. I liked the authors talks and livestreams on YouTube.

Lua!

Lua is 26 years old. The OP said younger than Go. Go is 9 years old.