As others pointed out, alternatives already exist. Besides Godot and ebiten, there’s also Stride, which focuses on 3D.
Now, an Open Source alternative that is compatible with Unity, similar to what EnigmaGM offers as a counter to GameMaker (and, in a similar vein, FPC Lazarus vs. Embarcadero Delphi), would require a fuckload of work and people with the skills to make it work.