I imagined just a bit of possibilities that this would enable and found it to be very scary. While this is a dream that most of us have, I think that it opens doors that are better sealed tight.
I installed Stylus straight away after malware was discovered in Stylish. But it doesn't mean Stylus is automatically safer because it's an alternative. Anyways just to be sure, I manually audit the source code of plugins before I install them. I typically do this by opening 10-20 .JS files in Notepad++ and inspect what all the files are trying to do. Firefox plugins are not compiled binaries so it's easy to extract them and then start inspecting the code.