I don't mind the invisible border and the "you cannot go this way". The only thing I miss is this border being shown on the maps, so that I can at least avoid the message. In some places in OB this border is also placed at very unnatural places. Like 200m in front of me there is this deep fjord that could have served as a perfect natural barrier, but instead I'm getting the message at flat passable areas.
Don't make everything unpassable natural obstacles, it would just feel unnatural
I mean, there could be roads going over mountain passes across the southern borders. You could have a natural indication of a border, like a bridge or river or elevated pass (that you could climb onto) or texture change (Bruma area changing to winter textures), without it being an actual physical barrier all the time.
So for me, a natural *indication* (rather than a natural barrier) combined with a "you cannot go this way", that is also marked on the maps, would be the least intrusive solution.
Arma2 (realism based combined warfare) has a different approach, in that at the edges the landscape is fully replaced with a fractal landscape without anything. So you can go on forever until floating point errors stops you. It also have skyboxes that you can always see but never reach. It could be a method, but it is highly dependent on how the landscape system works in the engine. Might not be viable at all.