Amigas used to be on top in terms of bang for buck, multimedia performance, OS stability, and nice form factor.
Since then, Amiga's competitors have invested billions in these areas, making it extremely hard for any company without similar means to beat them at their own game.
I still think there's room for a 3rd party in the PC market, though. Some areas of innovation that are in the Amiga spirit and might work for a smaller company could be a ray-tracing GPU, tightly integrated and easy to use OS/Apps (like iPhone), interesting user input devices like a touch-screen keyboard or wacom tablet, and built-in tools for easy graphical application development.
Edit: Condensed for the modern reader.
