Fair game is any sort of information that could be collected with some amount of time and effort, even if it isn't being collected at the moment. And it has to be legally obtained.
I'll take a stab at the question just to prime the pump. Suppose you formed a private online club of people who represented a random selection of investors in general. So you'd have some professional advisors and lots of regular folks who pick their own stocks. Every day at a set time the members of the club are asked to submit to the private online system their opinion of what stocks they find attractive to buy in the next 24 hours. Before the results are tabulated and distributed back to the members only, each member is given an opportunity to invest any amount of money in whatever the club ends up picking as the most popular stock. If they opt in, all of their trades would be executed at the same time, or perhaps randomly at nearly the same time, so no member gets an advantage. Only the single most popular stock would be purchased for all members, one per day.
If selected correctly, this random collection of investors would, on average, have the same opinions as the public in general, but they would execute their trades before the rest of the world, on average, and get in before the generally popular stocks run up. This concept banks on the notion that whatever ideas in the media or in life that are floating around influencing minds are influencing the member group in the same way as the public at large. Call it zeitgeist.
What's your take on the most valuable information in the world?