Back in the early days of the MMO, there were games in which game masters would occasionally come online to tell a story to the players that were currently playing. They would take control of NPCs and monsters to create a dynamic experience, and sometimes even hand out unique items to a select few players who performed admirably throughout the event.
In more recent years, it seems like companies stopped doing this altogether. Too many servers, not enough people would get to experience such events (leading to jealousy) and the employees spending their time on this cost money, which they would rather spend elsewhere.
To me, it's sad that GM-driven events have not been explored more. It's something that can easily make a subscription worth it, and allow a game to stand out from the rest. These events make for a memorable experience. It's something people would remember and talk about for a long time to come, websites will document the events and role-players in particular will feel that the world is just that much more alive.
"Do you remember when King Emeric was slain?"
"Of course. That's when we attacked that dunmer village at the Morrowind border that was harbouring the assassin. A week - and probably a few torture sessions - later, it was revealed that he was enlisted by a political opponent within Daggerfall itself, who was an agent of Boethiah. The traitor was unmasked and we brought him and his daedric allies to justice... by the sword."
Scripted events can only deliver so much.. they feel a lot more artificial.
"Do you remember when King Emeric was slain?"
"You mean in that quest with the dark elf assassin? Yeah, I did it on my alt yesterday."
The megaserver system eliminates some of the problems MMOs in the past might have had with this. Most notably that you had to have a character on a specific server to be able to participate. Another advantage is that the ZO in-game staff (most likely community managers and in-game tech support people, NOT developers as some people might expect) are not divided among servers, which means they'll have a team that's large enough to occasionally miss one or two people without risking being undermanned when it comes to helping people with their in-game problems.
On the other hand, the mega server introduces a new issue. Even with various "phases" of the megaserver and having the communities divided among 3 alliances and 2 megaservers (US and EU), it might get very crowded, making it difficult/stressful for a game master to interact with players directly. Therefore, it would most of the time only be feasible to tell stories with a predetermined outcome (maybe 2, if player combat is involved in a win-or-lose situation), and only let the players tag along to see it unfold and engage in combat when appropriate.
Examples of things that could be done:
A town crier could shout through a zone when something interesting is about to happen
An innkeeper might be sharing some gossip with another NPC in a tavern
Important NPCs (such as faction leaders, fighters/mages guild members and even known villains) might have a public conversation/confrontation with one another
Powerful enemies might attack a town, and players will have to take part in the defense
An NPC might show up to a group of players and ominously drop hints of things to come
Note that all of these things can be done with existing assets. It would require no time from the developers, who keep working on improving the game in other ways. That doesn't mean the events have to be inconsequential by themselves; they could be used to introduce changes to the game world or completely new content. For instance, if Zenimax Online wants a town to be overrun by daedric forces, there could be a GM event where this attack takes place in which the defending players and guards are ultimately destined to lose.
Anyway, I think there's plenty of ways this could work. Question is, would you like to see these sort of events return in TESO?