I'd like a PDA that ran emacs. And nothing else. Nothing. I'd have an email client, a news client, a web browser, an irc client, a calendar, a calculator, a psychologist, several games, a text editor and a programming language available all the time, in a familiar environment.
That would be The Perfect PDA(tm).
Feel free to take the idea to production - you can give me one for free when they start coming off the assembly line, but I won't make that an ultimatum.