I am part of the dev team for a public Terraria server. We host different minigames that players can play (Capture the Gem, Spleef, Team Deathmatch, etc.). We already have a well-established community, and you can check us out and get more info at our website, pedguin.com. The benefit of this is that you'll be working on a project that has a real audience and reaches thousands of players every week. There are lots of things we want to do in terms of adding new content, fixing bugs, growing our audience, and finding new ways to explore the possibilities of what you can do with a Terraria server. Having such a large community maintained by only a handful of active developers is not easy!

Tools we work with:

C#

Terraria's codebase

SQL

Frontend web development

AWS (We're in the process of migrating to this)

Things you could do:

Work on our many upcoming minigames. There are lots of ideas here.

Expand our server to include other features such as freebuild and survival.

Streamlining the user experience. We want our server to be as accessible as possible so that you can join and play with as little hassle as possible.

General maintenance and bugfixes.

Work on our website. As you can see from the above link, the site isn't very pretty and is in need of a redesign.

Anti-hacking, anti-cheating, anti-spam, and anti-ddos systems.

Upgrading our backend server architecture. We want to allow our server to scale more easily and support more players without running into lag issues. We hope to grow our audience a significant amount in the future, and this requires a migration to a distributed architecture.

As far as money, this is for the most part a hobby project. We make a small amount (around $200/month) from player donations, which is distributed among our developers based on their contributions. Not really enough to call this a paid project, but maybe you'll be able to buy the extra nice ramen after you've worked with us.