I had always this feature in mind for long time and the event in Uldah in which that GM controlled the Arcanist npc and spoke in a rp manner, could open a large variety of possibilities.
GMs (gms, community reps, devs, etc.) could control some special monsters or NMs sometimes and randomly (without previous warnings). It could become a sort of pvp in which we have to combat with a monster that have a real strategy in mind and not with a script. Like a Dragon that appears in the sky sometimes and roams around, not in a definite pattern but it (gm) goes where it wants. Attacks villages or hubs.. or simply some unlucky travellers. Obviously players can attack it and kill it (i can see great rewards here). So as other kind of special creatures.
In the same manner, controlling NPCs. It could appear a special Merchant that travels, with a carriage maybe, for a limited amount of time, from a city to another (different path every time). Or a Bard who plays music or tells stories in a city (everytime different.. and maybe he could put buff on you if you give him some money lol). And, like we saw, for world events.
And so on.. infinite possibilities. :0 Everytime you log in, you could be surprised by something.. (and also gms could have great fun to do this!)
What do you think about it?