Games like The Endless Mission
Games like The Endless Mission include Don't Starve, Kerbal Space Program, Terraria, Wobbly Life, and Garry's Mod. We matched them by the tags players give The Endless Mission on Steam (Indie, Adventure, Early Access, Sandbox, and Moddable): closest matches first, and among equally close ones the better-known games.

Don't Starve
Shared: Adventure · Indie · Sandbox

Kerbal Space Program
Shared: Sandbox · Indie · Moddable

Terraria
Shared: Sandbox · Adventure · Indie

Wobbly Life
Shared: Adventure · Sandbox · Indie

Garry's Mod
Shared: Sandbox · Moddable · Indie

The Scroll of Taiwu : Beyond The Dome
Shared: Sandbox · Indie · Adventure

while True: learn()
Shared: Indie · Programming · Education

Sands of Salzaar
Shared: Indie · Adventure · Sandbox

Automation - The Car Company Tycoon Game
Shared: Sandbox · Early Access · Moddable

Totally Accurate Battle Simulator
Shared: Sandbox · Indie · Adventure

People Playground
Shared: Sandbox · Moddable · Indie

Feed and Grow: Fish
Shared: Early Access · Indie · Adventure

CodeSpells
Shared: Early Access · Indie · Programming

Grey Hack
Shared: Indie · Early Access · Programming

Starbound
Shared: Sandbox · Adventure · Indie

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

Mount & Blade II: Bannerlord
Shared: Sandbox · Indie · Adventure

Blockland
Shared: Sandbox · Indie · Moddable

Project: Gorgon
Shared: Indie · Early Access · Sandbox

Hard Bullet
Shared: Sandbox · Indie · Early Access

Total Miner
Shared: Early Access · Sandbox · Indie

Human Fall Flat
Shared: Adventure · Sandbox · Indie

Beyond Sandbox
Shared: Sandbox · Moddable · Indie

Ravenfield
Shared: Sandbox · Early Access · Moddable