Games like The Nebula Project
Games like The Nebula Project include Terraria, UFO 50, Starbound, The Escapists, and Your Only Move Is HUSTLE. We matched them by the tags players give The Nebula Project on Steam (Action, Adventure, Strategy, Arcade, and Sandbox): closest matches first, and among equally close ones the better-known games.

Terraria
Shared: Sandbox · Adventure · 2D

UFO 50
Shared: Action · Arcade · Strategy

Starbound
Shared: Adventure · Sandbox · Pixel Graphics

The Escapists
Shared: Strategy · Action · Pixel Graphics

Your Only Move Is HUSTLE
Shared: Action · Strategy · Sandbox

Cyberhunt
Shared: Action · Adventure · Arcade

Mount & Blade II: Bannerlord
Shared: Strategy · Action · Sandbox

Mount & Blade: Warband
Shared: Strategy · Action · Sandbox

Reassembly
Shared: Strategy · Action · Sandbox

Forager
Shared: Adventure · Sandbox · Pixel Graphics

Totally Accurate Battle Simulator
Shared: Sandbox · Strategy · Adventure

Trailmakers
Shared: Action · Adventure · Flight

Exit the Gungeon
Shared: Action · Arcade · Adventure

SANABI
Shared: Action · Adventure · Pixel Graphics

SPORE™ Galactic Adventures
Shared: Action · Adventure · Sandbox

Noita
Shared: Sandbox · Action · Pixel Graphics

Don't Starve Together
Shared: Adventure · Sandbox · Survival

Broforce
Shared: Action · 2D · Pixel Graphics

Halo: Spartan Assault
Shared: Action · Top-Down Shooter · Strategy

Shovel Knight: Treasure Trove
Shared: Adventure · Action · Pixel Graphics

Cortex Command
Shared: Action · Strategy · Sandbox

Space Pirates And Zombies 2
Shared: Action · Strategy · Adventure

Edge of Space
Shared: Adventure · Sandbox · Action

Ultimate Epic Battle Simulator 2
Shared: Action · Strategy · Sandbox