Since the sandbox mode can just browse and manipulate a real repo, it would be fun to be able to open one of my existing repos in the Sandbox and use the game as a simple visual GUI for my own repos for fun. :)

That's a great idea! The reason why we've not yet implemented that is that the graph layouting algorithm we use doesn't perform super well for, let's say, more than 50 commits. But we could potentially improve that, or show a warning when people try it with big repos, so they won't be disappointed.