Godot is an advanced, feature-packed, multi-platform 2D and 3D open source game engine.

Create games with ease, using Godot's unique approach to game development.

Nodes for all your needs. Godot comes with hundreds of built-in nodes that make game design a breeze. You can also create your own for custom behaviors, editors and much more.
Flexible scene system. Create node compositions with support for instancing and inheritance.
Visual editor with all the tools you need packed into a beautiful and uncluttered context-sensitive UI.
Friendly content creation pipeline for artists, level designers, animators and everything in between.
Persistent live editing where changes are not lost after stopping the game. It even works on mobile devices!
Create your own custom tools with ease using the incredible tool system.

Limitations of the Flatpak version:

No C#/Mono support. See: https://github.com/flathub/org.godotengine.Godot/issues/8
Custom builds for Android don't work. See: https://github.com/flathub/org.godotengine.Godot/issues/63
The old-fashioned way of exporting to Android still works: https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_android.html
External script editors are supported, but you need to follow the steps described here: https://github.com/flathub/org.godotengine.Godot#using-an-external-script-editor

Godot