Everthing old is new again... and some ideas that seem old haven't actually been completed yet. Other ideas seem to fail, but only because they are "before their time".
With that in mind, I'd suggest going back and reading the writings of people like Doug Enbelbart, Ted Nelson and other computing pioneers (and not just computing pioneers, really, but any great thinkers. How about Nicola Tesla, for example?) and look for places where they proposed amazing things years or decades ago, that still don't exist, or don't exist as fully as they could. On that same basic note, go back in time and re-read some old issues of Infoworld, Computer World, Information Week, Business 2.0, Red Herring, Fast Company or Wired from the late 90's or early 2000's and mine for "before their time" ideas that might be ripe for a second shot.
Hanging around hackerspaces is also a great way to gain exposure to a constant stream of interesting ideas and approaches.