Finished Terraria and looking for something similar? Let's take a look at a few similar games, all of which support co-op, as we'll be getting the boys to join along.
1. Terraria's Calamity Mod

Calamity isn't really a separate game, but a content mod. In fact, it's the biggest Terraria mod to date that's also in active development. It turns the game into something completely different.
It adds dozens of hours of endgame content, enemies, bosses and items throughout the game's progression. I personally have completed this mod at least 15 times. The replayability value is crazy, because Calamity comes with it's own class system (including it's own custom class - the Rogue class!).
There's plenty of progression guides, but the official Class Setups Guide is generally considered to be the most updated one.
Average playtime: 100+ hours on a first playthrough
2. Starbound

Starbound is essentially Terraria in space! The game also features different races, dungeons and a whole planetary system to explore. Planets are procedurally generated and have different biomes and creatures, so you'll always have something interesting to explore.
The progression feels similar - you mine resources, craft better gear, and fight tougher bosses. Unlike Terraria though, there's a story progression in this game (it's pretty interesting).
Average playtime: about 60 hours if you're following the main story.
3. Core Keeper

Core Keeper is a relatively newer game compared to everything else in this article. But don't let that fool you! This game is kind of a mix between Terraria and Stardew valley, but in a very interesting way.
It has a hunger and thirst system, a very interesting take on combat, and very challenging bosses. This game require you to come up with actual strategies to progress. There's quite a lot of unique mechanics including interesting dungeons, farming, merchant NPCs, deep skill trees and cooking. That's right, COOKING.
Personally recommend it. You will like this game.
Average playtime: 75+ hours, but 150+ if you want the legendary weapons!
4. Necesse

Necesse is also pretty similar to Terraria. The main difference is the top-down perspective instead of side-scrolling.
There's more emphasis on farming and town development, because of the advanced NPC system. The combat system uses WASD movement with mouse aiming, which is a little different than Terraria.
The progression system is also quite different, using skill trees and level-ups rather than just item-based advancement. Just like Terraria, Necesse has the multiple biomes, boss fights, plenty of stuff to build and a mana system.
Average playtime: just about 25 hours
5. Valheim

Valheim is similar in the sense of exploration, fighting bosses and equipment progression. Bosses and enemies in this game, however, are very difficult and you are likely to die a lot until you learn how they work.
The procedurally generated world is gigantic - you can spend real life days exploring it. The building system is very polished, you can build gigantic castles with lots of details.
If you don't mind that it's 3D, you should definitely check it out. It's a very fun game to play with friends and features some unique mechanics like portals, magic and boats (one of my favorite parts):

I've personally played Valheim twice (with a few mods), and it was a blast. The developers are actively working on it, with the Ashlands Update coming out last year, and the Deep North update being scheduled for early 2025.
Average playtime: 150+ hours, this is a difficult and very long game!
6. Conclusion
That's about it! There's plenty of good similar games to play nowadays. If you have another game in mind (or you're making one yourself 🗿), we can add it to this post. Feel free to join our discord and tell us about it.