I can't speak to the complexity of building an open source Figma. The UI isn't that crazy. But even if it existed, you would need a cloud storage solution. And

I can't speak to the complexity of building an open source Figma. The UI isn't that crazy. But even if it existed, you would need a cloud storage solution. And the file sizes can be quite big. And then there's the ability to share project urls, so you'd need some sort of hosting setup.