To the east, mountains. We know this.
To the south, mountains. We know this.
To the west, mountains. We know this.
To the north, ice cold oceans. We know this.

Not rocket surgery.

But yeah, if they add in levitation, probably invisible walls.

I'm not sure exactly what the borders are like but to me it looks like based on the shape of the western border and the little opening north of high rock that there's a river running north there. That's one border. You would need invisible walls or impassable currents in the river, but they aren't as obtrusive, and all the bridge crossings could be closed off by the army.

To the east there's obviously a huge mountain range, any of the passes could be blocked off by recent rock-slides or also closed down by the army.

The north is obviously the ocean.

To the south is the only part that might be a problem to avoid invisible walls. Personally I'm fine with them though.

I think it would be better to actually show that routes out exist, even with invisible walls, rather than block everything up with mountains and water, since other people do travel and its only the player that needs to be stopped. Where would people get their news from the other provinces if Skyrim is completely isolated?

I guess they could have the exits mostly covered by mountains, with the roads leading to mountain passes with a wall and a big door. If you try to go through the game asks, "Abandon your quest and leave Skyrim to its fate?", and if you say yes you get the bad ending and its the only way the game ends.

They should have a http://upload.wikimedia.org/wikipedia/en/e/ea/Skifree_yeti.png that rapidly catches up with you and eats you...

To be honest, I actually don't mind invisible walls. Oblivion is generally good at handling this, as the boundaries are often marked by sudden sheer drops, or cliffs, or other noticable changes in detail. The only time it becomes annoying is in the certain places where it is not at all obvious you'll bump into the wall. I think if they just had the terrain change noticeably along the entire boundary of Skyrim, it would be fine. You wouldn't have the problem of trying to escape and hitting the wall, that occasionally happens in Oblivion, as you would be able guess where the boundary is.

The only thing I do hate is the rather blunt "You cannot go that way - turn back" message. Personally, I think it should be "I shouldn't go any further" or something along those lines.

I think you should be able to go far out to the ocean, but for the rest I think there should be:

1) either natural borders, like huge mountains. If levitation exists in the game, there should be something close to these natural borders that stops you from casting spells. You could blame it on the cold, perhaps?

2) or either no boundaries at all. You should be able to travel like you could in Oblivion with invisible walls turned off. There will be trees, grass, bushes, but nothing else.

I don't like the invisible wall.

I'm torn about the issue of invisible walls. On the one hand they can be somewhat annoying to run into even if they can be removed somehow (assuming that there is a "construction set" that comes with the game) while on the other they fr have the potential to allow for a far greater level of environmental diversity. The diversity of Oblivion's landmass was limited by the view-distance; having too drastic a difference between any two areas of the map (such as a rain forest in the south) would appear ugly from a distance. This could easily have been avoided if the landmass had been divided into separate regions, between which fast travel was possible (exactly how this would have been implemented is irrelevant). However, this makes it necessary to have invisible walls surrounding the disjoint landmasses. The benefit of having disjoint landmasses is that it not only increases the level of possible environment diversity, but also increases the potential for mods (again, if the CS is included) to fill out the rest of the world.
Environment diversity can be anything from diversity of political intrigue to diversity of climate. There would be the possibility of having two or three warring factions in the game, each with their own "capital cities" and outlying territories without worrying about artificial intelligence causing potentially gamebreaking situations.

Now briefly about the potential pros/cons of including the CS in the game:
Pros: modders are amazing, creative, and in many ways responsible for the improvements between successive games, and having the CS continues this.
Cons: although modders can add a lot to the world, the CS itself limits the diversity of the environments. Consider for instance the landscaping tool, which is incapable of making rugged terrain such as cliffs, overhangs, or canyons.

Anyways, this post is not specifically pertinent to ES V, since the game is already well underway (and is probably going to be just fine)

lol so your asking for a whole underfryte race running around skyrim?

Pretty much.

I'm guessing something like this is the most likely.

u know how in those like dirt bike or 4 wheeler games when u go outside the map u just get launched like 2 miles back in to the map without dieing thatd be cool i guess?

When you walk past the border, a message pops up and says "You are not prepared to travel out of the province". At that point you begin to lose fatigue. Once your fatigue reaches zero the screen blacks out and you wake up at the nearest inn.

A gravelly-voiced Dunmer says "Wake up, you were dreaming".

What about mountains yes but then where there aren't mountains there would be a big wall with a gate, being that Imperial Crossing Border.
There would be a squad of Nord/Imperial Guards shouting "Stop Right there, criminal scum" or "by the gods there is a psychopath on the loose!" every time you try to go through it?
Then if you choose to enter the gate it would be locked

When you leave the Provence a waring will come up and if you ignore it a assassin or the spy backstabs you. :dead:

The ocean should be an ocean. The more ocean the better. There can't be too much of it. Miles and miles of ocean even if it is nothing but ocean.

Think about modding isles into the great wide open...

If it's like Oblivion and Fallout 3, I wouldn't mind it. Don't make it like New Vegas: the invisible walls in that game were truly terrible, just because they were everywhere. Want to climb this mountain pass? You can't, there's an invisible wall here.

Oblivion and Fallout 3 at least gave you the sense that the world was a lot bigger than what you were seeing. I only ran afoul of the invisible walls a few times. New Vegas' playable area was entirely boxed in. Seriously, they even had fences in the water.

You have to remember that the part of Morrowind that you play in is Vvardenfell, an island. While in Oblivion, you play in Cyrodiil, not an island.

if you go to far, the all mighty Todd Howard will slay you