AI could do it every outcome is different on each realm.
Example PvP/PvN as killing someone or npc around a city with no witnesses. The AI for the realm will generate a quest to find said players with clues. Then end result bounty/wanted posters around area or where ever a player would like to put them giving another quest.
Example PvE the realm starts with a 1000 dragons 500 male/500 female. Max amount of dragons is 2k and players decide to kill some due to adventure or gold it will have to take time reproduce or you can wipe them out. If dragons are something major as giving the realm magic and you wipe them all out AI will create effects for that realm. Then give quest for new power source or ways to bring back the dragons.
PVE destroying a city npc will try to rebuild other cities. So fourth.
You could make a realm for about 50 players to start and each realm could hold thousands. Then be able to travel you have figure it out. Realms are worlds each one starts the same. Now by the time you travel to another one it could be very different as humanity is wiped out, no magic just mechanics, over ran with demons, undead, dragons, over populated, and so forth. Then add in stuff as you go.