I used to think that no new ideas could be thought up.

Then I had one, about 1991 or so, completely by accident:

Humans are the sex organs of the planet

It's a simple idea really - if Gaia is really a life-form or an individual organism (as per The Gaia Hypothesis), then she must be able to reproduce, since that is the one function of life-forms she has yet to display. (The others are nutrition, growth, decay and evolution.)

I, like you, thought "someone else must have thought of this before me" but after years of looking, I can't find the evidence. As far as I know, it was completely original with me.

Of course, it will only prove true if humans start buidling ecologies in space or on other planets...at which point, Gaia will have reproduced...becuase of us (hopefully).

BTW - it's a well-established fact (in the business-major community) that world-changing ideas occur about once every century per 100,000 people. With our current population, humanity sees a new one about twice per day. That's why the future seems closer now than it used to, and society seems to evolve so fast.

So, don't give up - you might be the next person to have one, and new ideas are the life-blood of our civilization, even if some of them are bullshit...for a while.