Build big, stupid custom spaceships with friends in Cosmoteer
Or make functional ones, whatever, I'm not your dad.
Space, strategy, sandbox, and building nerds, have you seen Cosmoteer yet? It's a starship designer-simulator that puts you, or you and some friends, in a big universe to command an ever-growing custom built fleet against a variety of AI or player opponents.
It's a cool loop, one where you defeat enemies, complete contract missions, and go mining to get resources, then use that loot to upgrade your ship so you can take on ever-harder tasks. You can do all that with buddies in co-op. You can also test your ship designs against players in PvP, and make wild ideas work before you invest in them via the creative mode.
It's a pretty dang solid game for Early Access, probably because it's one of those cult classics that has been in development for a decade and was previously only available via the developer's website. That developer is indie outfit Walternate Realities, by the way, and the guy who runs it is Walter, and I'd like to congratulate him on a superb pun name.
In the future of Early Access Cosmoteer intends to add carrier ships with drones and fighters, boarding enemy ships, hinged joints for transforming ships, as well as a variety of new weapons and ship modules. It'll also add new mission types for the singleplayer and cooperative mode, and additional modes for multiplayer PvP.
Cosmoteer is a bit easier to get going on than some more complex 2D-space-constructor type games, which is nice for that cooperative angle. Once you get into the weeds of design, however, you get to do some neat stuff. Giant mid-century sci-fi spinal mount weapons are available for ships, requiring you to build the whole thing around one giant gun. There's also details like ships so large they have their own ammunition factories onboard, so you can gather resources and refine them into shot mid-mission.
To me Cosmoteer is one of those titles that by all the logic of small game development should not have a functional multiplayer element, given the modular destruction and customizability and thousand other ways it could desync and break. But it does, in both cooperative and player versus player flavors, and that's definitely part of the charm.
You can find Cosmoteer on Steam for $20, where it currently enjoys a somewhat-shocking 96% positive out of 1,113 reviews.