The mage-trainer's room is actually below Stormwind. There was a time pre-Cata when you could go under SW and there was a hole where the tower was above. If you jumped into the hole you'd either fall forever, die, or land in the room.

Lore-wise.... who knows?

Bolvar is in hibernation using all his will and power to control the Scourge remnants. Last thing we need is the Old Gods, Twilight Hammer, Deathwing, and Hogger (couldn't resist) mixed in with some uncontrolled undead running around.

Dalaran is dealing with extensive cleanup after having a million mounts from both faction's mounts pooping everywhere.

Dalaran would probably move to Outland. Along with anyone else who wanted to survive.

Number of Old Gods? Zero.
Number of insane dragon aspects? Also zero.
Number of meddling Titans? Yep, zero.
Number of Lich Kings? Like, only seven the last time I checked.
/double checks. Oh yeah, I meant zero.

In contrast, you've got boat loads of Naaru trying to heal the place, blood elves and draenei *presumably* working together after the Sunwell Plateau / Velen thing, and a chance to pull the planet back together again so there's room for everyone.

Sylvanas has never shown the capability to leave her body and possess people now. I don't think she'd want to, she can disguise herself pretty easily through other means, and I imagine she's horribly paranoid/afraid of losing her body again after being unwillingly ripped from it when she died.

Dalaran was probably crashed into the ground in the Crystalsong Forest, Deathwing likely just sat on it until it fell. If the other major cities fell to send the spirits of their leaders there, pretty sure Dalaran either ran away again (and that would be cowardly but not shocking) or fought one last hurrah and crashed and burned.

@Hob
I'm slightly skeptical about Dalaran's ability to fit through the Portal...