What are some alternatives?
When comparing Tombstone Engine and Superpowers, you can also consider the following products
Armory3D - Armory 3D is a free and open source game development software developed using Haxe and Kha and made to run inside Blender.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Urho3D - Urho3D is a lightweight, cross-platform rendering and game engine implemented in C++ and released...
Panda3D - Panda3D is a video game development engine and rendering software. It is developed and maintained by the Carnegie Mellon University Entertainment Technology Center.
MonoGame - MonoGame is an open source implementation of the Microsoft XNA 4 Framework.