For Urge, I'd be verrrry disappointed if a deus ex machina was employed so that the PC would fight a dragon the size of Nirn. The combined efforts of Mehrunes Dagon (as the Leaper Devil King) and Shor only caused him to get too full to eat everything.

Visually, I think I would rather have him be a massive Dragon, wreathed in smoke and magma, pocked with craters and frozen methane, visible in the sky behind the clouds beside the moons, with a smaller avatar that the player has to fight at the end in a specific place in order to keep his sky-avatar from manifesting fully.

OK this is complete speculation on my part.
Alduin is not actually portrayed as a dragon rather a firestorm or apocalypse of creation.
Lorkhans heart was at the center of the world, the events surrounding it led to Red Mountains effects.

Complete BS again here on my part, but could not a "world eater" of fire, represent Seismic activity..
Volcanos, firestorms earthquakes, magma pouring out, oceans boiling and flooding, just as the creation of a planet.. hense destroyer and genitor.
This type of activity would wake up a bunch of dormant dragons, causing them to head out.

Rofl maybe the goal of Skyrim is to replace Lorkhans heart in the earth, by tearing it out of your own Shezzarine chest.

Mundus is not Dagon's birthright, only a place to utterly annihilate. Mundus is the realm created by Lorkhan/Shor/Shezarr along with the aedra, who may or may not have liked the idea, depending who you asked. Don't believe everything Mankar said, for a bunch of stuff is incorrect, while some of it is correct, to an extent.

Ummm...they already showed us http://www.dragoart.com/tuts/pics/8/2729/how-to-draw-cassie-from-dragon-tales.jpg