When the 4th edition announcement came I was working on my new campaign world. The concept was then "a world gone mad" where to make it simple it was on its way from something like Toril to something like Athas (FR and Dark Sun, for those less geeky). The important sources of power would come from the fey and the shadow realms, and a fey like race would be the rulers.
And what do they announce? Eladrin. Feywild. Shadowfell. Of course I had to move to 4th edition with this, and try to rework it so that it would be a good intro to the new edition for my prospective playing group.
More things have changed over time, and as at least one of those players reads this forum at times, I won't say too much of it. Some little things:
I have focused on one continent, which is a kingdom ruled by the Eladrin. The continent used to have a population of several millions, now there might be about 500 000 as a whole. Natural disasters, plagues, famine, monster raids and wars have claimed more victims than anyone wants to think of.
All the PH races and classes are suitable for players. Apart from openings to Feywild and Shadowfell all travel to other worlds is these days impossible for some reason.

Apart from player races two intelligent and reasonably civilized races are known to exist - one insectoid, one undeadish.
Most halflings live on barges, travelling the rivers and what is left of the canal system. Elves and dwarves mostly keep to their respective traditional home areas, since that seems the best way to survive these days. Eladrins rule the few cities that have survived, and also live a lot in the Feywild. Humans do most of the farming and least of the decision making. Dragonborn have lost their old kingdom and now are spread out through the land, often working as guards and mercenaries. Tieflings are generally despised, seen as thieves and murderers who are probably responsible for everything bad that ever happened. (And that is a lot!) Half-elves are generally mixed in with the elves and the humans, not really seen as having their own culture.