With mods

A single-player open-world chaos sandbox with a historic multiplayer mod built for huge servers.

Solo by default. The multiplayer mod is famous but much quieter now, so check server activity first.

The Steam page describes extending Just Cause 2 to dozens, hundreds, or thousands of players; current population may be low.

Source: Just Cause 2 Multiplayer Mod

A solo open-world destruction sandbox with community multiplayer modes for freeroam, racing, and roleplay.

Solo by default. JC3 multiplayer exists, but server health varies and may be quiet.

JC3MP opens the game to community servers and custom game modes such as racing, deathmatch, roleplay, and freeroam.

Source: JC3MP

A solo RPG that can become a shared sandbox adventure with co-op mods.

Solo by default. Co-op is modded, setup-heavy, and best with a small committed group.

Skyrim Together Reborn is widely documented as an 8-player co-op option, but quest and mod compatibility still need planning.

Source: Skyrim Together Reborn

A solo deckbuilder that can become a lightweight shared run through co-op or race mods.

Solo by default. Multiplayer depends on community mods, so everyone needs the same setup.

Together in Spire advertises Steam or port-forwarded lobbies for up to 10 friends. Good for mod-tolerant roguelike nights.

Source: Together in Spire

A single-player space-program sandbox that can become a shared universe through Luna Multiplayer or DarkMultiPlayer.

Solo by default. Multiplayer depends on a server and agreement around time warp.

Luna Multiplayer and DarkMultiPlayer provide shared-universe server play, including time-sync/warp handling.

Source: Luna Multiplayer

Scrappy physics horror extraction for groups who enjoy panic, betrayal, and bad decisions.

Native lobbies are capped at 6. Larger rooms depend on community mods and can be less stable.

Community lobby-size mods can push past the native cap; treat this as best-effort rather than guaranteed support.

Source: community lobby-size mods

A colony story generator that becomes a shared chaos machine with multiplayer mods.

Solo by default. Multiplayer requires community mods, matching mod lists, and a patient host.

RimWorld Multiplayer says any player count is possible but recommends a maximum of 8. Best treated as a long-running modded campaign.

Source: RimWorld Multiplayer

Survival exploration that turns into a shared expedition through co-op mod setups.

Solo by default. Co-op requires mod setup and can have progression or sync rough edges.

Nitrox advertises large-event support up to 100 slots, but normal friend groups should expect a smoother experience at much smaller counts.

Source: Nitrox

A single-player metroidvania that can become a shared exploration or randomizer night through HKMP.

Solo by default. Multiplayer is community-made and best with a group willing to use Hollow Knight mod tools.

HKMP lets players host games and allow others to join their adventures; use ItemSync/randomizer notes for co-op-style runs.

Source: HKMP

A physics driving sandbox that becomes a casual group driving or racing server through BeamMP.

Single-player by default. BeamMP requires its launcher and server browsing outside the base game.

BeamMP is an active multiplayer mod with public servers and a large installed server ecosystem.

Source: BeamMP