I hasn't given an example of what I envision a world event to be. So I spend an hour to make one! :D
I planned a centaur attack on Ascalon Settlement. I am going to guess that around 400 players from each server/zone would participate.
Keep in mind that centaurs in the game are fairly weak units. My necro can kill 10 of them by herself. So I will give the centaurs 1,000 men. So how can these weak units win against the powerful humans?
Here is my battle plan.
At the beginning there will be three main battle zones: Left, centre and right.
Humans will spread out their ranks in an unknown formation and unknown numbers across each zone. However for this demonstration I would assume 100 humans on the left, 200 humans in the centre, and 100 humans on the right.
The centaurs separated their 1,000 men into 4 groups: 200 on the left, 200 in the centre, 300 on the right, and 300 elite troops in reserve at the back.
The battle follows a series of steps and plans:
1) The beginning would be a series of small skirmishes along the human ranks from all 3 zones. The centaurs' forces in centre zone and right zone are used to delay the humans, keep the humans busy and hold the humans there. So their attacks are fairly easy to dealt with. In fact the centaurs there will try to preserve their forces until later.
2) The main attack begins on the left zone. In the heat and chaos of the battle, the centaurs would suddenly rush the 3 elite units from the back to help the attack on the left zone. The combined might of nearly 200 regular and 300 elite centaurs should be able to overwhelm the 100 human defenders.
3) The battle moves on quickly after that. The centaurs, instead of attacking Nebo Terrance, swings around south of the town and attacks the centre zone humans from behind. The surprise factor and sandwich effect of this flanking maneuver should crush the humans.
4) The centaurs would leave a small force at the west gate of Ascalon Settlement to guard it (not attack). The main body however moves south-east to take over the southern hills of the town. The dead humans, now resurrected from a nearby Asuran gate, makes a stand there. I don't really expect the centaurs to win this battle here. They probably lost most of their men by this point. But assuming that they did...
5) The centaurs leaves a small force to guard the southern gate of the town. The rest would move north to attack the south side of the humans in the right zone. Once again the humans are sandwiched.
6) If the centaurs wins, they moves to the northern heights and eastern heights to besiege the town with siege weapons. The centaurs, unfamiliar with siege warfare, decided not to attack the town walls directly but instead hopes to starve the town into submission. By this point, all exits in and out of Ascalon Settlement are blocked off by centaurs, so no supply can enter.
7) The players have 2 days to organize themselves and fight off the centaurs, or Ascalon Settlements will be destroyed and looted.
As you can see, even with all these planning, it will be tough for the centaurs to win. Like I said step #4 will most likely be a human victory. But the centaur gave the humans a semi-beat down.