What would change everything is not even a thought. It's more of a feeling.
Human development thus far has been fueled and guided by the feeling that things could be, and are probably going to be, better. The world was rich compared to its human population; there were new lands to conquer, new thoughts to nurture, and new resources to fuel it all. The great migrations of human history grew from the feeling that there was a better place, and the institutions of civilisation grew out of the feeling that checks on pure individual selfishness would produce a better world for everyone involved in the long term.
What if this feeling changes? What if it comes to feel like there isn't a long term—or not one to look forward to? What if, instead of feeling that we are standing at the edge of a wild new continent full of promise and hazard, we start to feel that we're on an overcrowded lifeboat in hostile waters, fighting to stay on board, prepared to kill for the last scraps of food and water?
Many of us grew up among the reverberations of the 1960's. At that time there was a feeling that the world could be a better place, and that our responsibility was to make it real by living it. Why did this take root? Probably because there was new wealth around, a new unifying mass culture, and a newly empowered generation whose life experience was that the graph could only point 'up'. In many ways their idealism paid off: the better results remain with us today, surfacing, for example, in the wiki-ised world of ideas-sharing of which this conversation is a part.
But suppose the feeling changes: that people start to anticipate the future world not in that way but instead as something more closely resembling the nightmare of desperation, fear and suspicion described in Cormac McCarthy's post-cataclysm novel The Road. What happens then?
The following: Humans fragment into tighter, more selfish bands. Big institutions, because they operate on longer time-scales and require structures of social trust, don't cohere. There isn't time for them. Long term projects are abandoned—their payoffs are too remote. Global projects are abandoned—not enough trust to make them work. Resources that are already scarce will be rapidly exhausted as everybody tries to grab the last precious bits. Any kind of social or global mobility is seen as a threat and harshly resisted. Freeloaders and brigands and pirates and cheats will take control. Survivalism rules. Might will be right.
This is a dark thought, but one to keep an eye on. Feelings are more dangerous than ideas, because they aren't susceptible to rational evaluation. They grow quietly, spreading underground, and erupt suddenly, all over the place. They can take hold quickly and run out of control ('FIRE!') and by their nature tend to be self-fueling. If our world becomes gripped by this particular feeling, everything it presupposes could soon become true.