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

A city builder that can become a shared planning sandbox through the CSM multiplayer mod.

Solo by default. The multiplayer mod syncs many systems, but not every city simulation detail.

Cities: Skylines Multiplayer provides a client-server experience for users to build together in one city.

Source: Cities: Skylines 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 classic solo-first Souls journey that can become a more continuous co-op run through Seamless Co-op.

Native co-op is limited by summoning and boss boundaries. The mod is a better fit for planned friend playthroughs.

The Dark Souls Remastered Seamless Co-op mod is reported as synchronising progress and supporting up to six players total.

Source: Seamless Co-op

A harsh open-world action RPG that becomes a continuous co-op pilgrimage with Seamless Co-op.

Native co-op is session-based and restrictive. Seamless Co-op uses separate saves and its own connection flow.

Seamless Co-op is the established modded path for a start-to-finish shared run and removes many native summoning restrictions.

Source: Seamless Co-op

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

A solo sword-fighting game with an older online mod for PvP and small co-op experiments.

Solo by default. Sekiro Online is older and more experimental than the later Seamless Co-op projects.

Sekiro Online adds PvP/co-op style online items, but treat it as a tinkering pick rather than a reliable group-night anchor.

Source: Sekiro Online

A single-player mech campaign that becomes a squad mission night through the Armored Core Coop Mod.

The base campaign has no co-op. This is unofficial and best for PC groups comfortable with separate launchers.

Armored Core Coop Mod adds unofficial online co-op and community reports describe up to 6 players with enemy scaling.

Source: Armored Core Coop Mod

A dense colony sim where a shared-base multiplayer mod lets multiple players coordinate one colony.

Solo by default. Current multiplayer projects are alpha/WIP and do not sync every system yet.

The Steam Workshop alpha says all players share controls over one colony and lists sync limitations, so keep expectations modest.

Source: ONI Multiplayer Mod

A two-player boss-rush classic with community mods that expand the local co-op count.

Native co-op is two-player. Extra-player mods are local and niche.

Community mods expand Cuphead's local co-op limit to 3 simultaneous players.

Source: Cuphead 3 Players Multiplayer co-op