How about no borders? I de-activated Oblivion's and there was still lots to find till hitting the actual end. However, I'm for some sort of message that would inform you that the actual, developed world, is coming to an end soon.
But that would defeat the whole point of having inaccessible land in the first place: so we don't see an empty void.
HOW WOULD YOU FEEL if they stopped the running animation when going towards the edge, therefore making it appear you are stopping intentionally?
I'd like this, as it would provide a subtle clue that you're approaching a border, and you won't just stop dead in the middle of the wilderness (which is the main immersion killer people seem to dislike here). It would have to be
very close to the border though, as it would get annoying if you're trying to flee from enemies and the game is forcing you to move slowly.
As for the "endless ocean in the north of Skyrim" ideas that have been thrown about; it wouldn't work as you would still need borders on the far left and far right sides of the map. Otherwise people could essentially swim around the borders and end up seeing empty voids and floating landmasses and whatnot.