I think there will always be some independent nation states that will refuse to integrate into a greater union. The current EU fiscal debacle could put the dampeners on further monetary unions for another century or more. Even the political dissidents in places like Russia and China have no interest whatsoever in compromising their nation's sovereignty and that's unlikely to change even in several generations. Why should they?
There are also many potential sources of conflict coming down the pipe over the next few hundred years. The oil will eventualy run out completely, which should make the Middle East even more interesting and fun than it is now.
As more people in the developing world add more meat to their diet, the pressure on arrable land will rocket and take global food prices with it. That's already happening. The move to biofuels won't help that trend much either, so as some countries develop further this will drive others into an even deeper poverty trap.
Religious differences show no signs of softening, ever. If anything, increased acess to communications and travel exacerbate cultural differences as much as they erode them, so that moderate demographics move to the centre, but extreme demographics move even further to the fringe and become estranged from moderates within their own cultural/religious group.
Nuclear technology is slowly but steadily proliferating. The genie is well and truly out of the bottle on this one. At this stage, eventual nuclear terrorism or a nuclear incident betwen third world rivals seems almost inevitable in the long term. Has anyone else noticed how it's the well educated and technicaly minded ones that tend to go all human-fireworks?
A few hundred years is a long time to either sort these problems out, or run out of many resources and enter a long period of continual economic and environmental decline and stagnation. Just look at Japan. Once the wave of the future, now they're a long term economic basket case.
Technology is the key. What technologies come along to ameliorate our coming environmental and resource constraints will make a big difference. I also haven't discussed the many and varied mega-catastrophes that could easily kock the whole planet on it's arse over that kind of time scale. In a quarter of a century it's almost inconceivable we won't get hit by something at soem point. Bet it another Krakotoa, a Tunguska somewhere heavily populated ow worse, over the sea),
Cumbre_Vieja tipping over, the San Andreas Fault cracking open again. Oh, check this out. Just found it:
Mega Disasters.
Now I'm a fairly optimistic chap. Glass half-full and all that. I'm sure lots and lots of realy good things will happen in 250 years. I think it may well be a better time for most people t live in than the world today. However it's not all going to be plain-sailing and ups and downs along the way will cause plenty of political problems that aren't going to be easy to solve.
Simon Hibbs