Games like Onset

Screeps: World is an open source MMO strategy sandbox game for programming enthusiasts. You control your colony by writing JavaScript code that operates 24/7 in the huge persistent online open world.

Grey Hack is a massively multiplayer hacking simulator game. You're a hacker with full freedom to act as you wish on a vast network of procedurally generated computers.

Define your own destiny in this dynamic, open-world sandbox MMORPG. Explore vast fantasy worlds, fight monsters and other players in dynamic PvE and PvP battles, gather resources, set up a trade empire, grow your virtual home to a major city or conquer others. Chose your own way!

ArcheAge: Unchained removes the barrier between you and the ultimate MMO experience.

Shape your citizen, forge your destiny and sculpt your world in this epic voxel sandbox MMO built on endless possibility.

Playcraft is a multiplayer game-creation sandbox accessible to anyone!

Just Cause 3 Multiplayer mod is a free fan-created project bringing an explosive and highly-customizable multiplayer.

Tactical Craft Online - MMO sandbox with tactical role-playing elements with a large open fully destructible world, with clans, trade between players, construction limited to the players imagination, various crafting, mining of multiple ores, farming, fishing, a massive tech tree etc.

Steam Hammer is the first hardcore sandbox RPG in a steampunk setting featuring survival elements. Here you'll find all the advantages of sandbox games in the mysterious world of steam engines.

In a Massively Multiplayer World where you can fight, craft, explore and live, Arcfall offers a game style that captures old school RPG games with modern quality. Emerge yourself in a world where you can craft your own weapons and armor. Own your own house and fill it with furniture that enhances your character.

Modbox is a multiplayer game creation sandbox for Desktop+VR

A multiplayer survival game of parenting and civilization building. Get born to another player as your mother. Live an entire life in one hour. Have babies of your own in the form of other players. Leave a legacy for the next generation as you help to rebuild civilization from scratch.

A free-roaming squad based RPG. Focusing on open-ended sandbox gameplay features rather than a linear story. Be a trader, a thief, a rebel, a warlord, an adventurer, a farmer, a slave, or just food for the cannibals. Train your men up from puny victims to master warriors.

A fully moddable city sandbox with jobs, NPCs, apartments, garages, gangs, and more. Broke Protocol is an open-world action game focused on roleplaying and custom content. Define your goals and identity in a persistent, reactive world where you write your own story.

Mechanica is a multiplayer open-world survival game involving building and programming. Create your own bases, defenses, manufacturing systems and more. Explore the map and expand the scope and complexity of your creations.

The ultimate space sandbox. Participate in epic fleet battles, form alliances, strive to dominate entire galaxies and harness the universe’s resources for your industrious empire or the destruction of others. Customise your experience, the universe is yours!

The only aim in Rust is to survive. Everything wants you to die - the island’s wildlife, other inhabitants, the environment, and other survivors. Do whatever it takes to last another night.

3079 is a futuristic, open-world action first-person role playing game. All areas, buildings, items, quests and characters are randomly generated. You will find unique lands & challenges. I hope you enjoy it!