Dystopia. Overpopulation will be the causative agent. While I don't wish to play the role of the prophet of doom, there's little that can be done from an idealistic perspective. The population will double by the year 2050. Maybe we can deal with that, but then it'll double again by the year 2080, if the projected growth stays the same. Why won't it? We live longer and are able to have more children. Having children is programmed into us and it's hard to control such a thing, even in developed countries.
The U.N. has plans to limit overpopulation in Third World countries by education and other methods. Their projected goal is to level off the population at 7.8 billion by the year 2050. The Third World countries claim that it is Western Imperialism at work, that the problem isn't too many people but the unfair distribution of wealth in this world (they're wrong). Quibble, quibble, quibble.
One can analyse the effects of overpopulation and probably trace it back to almost every human problem, but here the problem is simple: greater demand than supply. We're all used to having air-conditioning during the summer. Imagine if you had to sacrifice that for 3 hours each day. Perhaps you'd grumble, but you'd do it. But if you changed hours to days, this would make things hard to take. Let's also assume gasoline ran out, traffic was at a gridlock, and you didn't have running water to bathe; barely tolerable. No food or drinking water; anger. Further imagine a certain sect of (say the rich and the influential) people somehow getting some of the amenities and you don't; a revolt will follow. This is the future. My vision of dystopia might seem like blather, but I've seen it and experienced it in very mild doses.
In most cities in this country, we're far away from the situation. But if the population of NYC or Washington, DC doubles, don't you think the infrastructure supporting life in the city will collapse? And the above situation will arise? How about when it quadruples? If you don't believe my prediction, then I'll tell you this: such a thing is happening in least one or two places in the world as you read this.