They should invent this thing that is like a virtual world, where it models, like, everything. Then you can run a simulation with just one person or a group of people to see what would have happened if you had done such and such different. You could do everything from "well what if I had tried to kiss her after all" to "what if we had just let Al Gore be president instead".
Actually, now that I think about it, that would just lead to more frustration and slapping of foreheads. We would live in regret constantly, and probably just end up ruining the rest of our lives.
Ok, so new rule: You can only use it before you make a decision, not after. It's almost like a magic 8 ball, except it really works. And it doesn't just tell you what to do; it shows the outcome of different decisions and then lets you decide. "Hey new invention thingy: what happens if I vote for boring Al Gore over crazy George W. Bush?" And who knows, maybe we would learn that Al Gore would have started two really pointless wars that resulted in thousands of deaths.
That would probably make life really boring, eh? Or maybe it would make life perfect. If only I could ask the new invention thingy what life would be like if I had it.