I suppose you could remove the instance, invisible barriers, and spawn mobs throughout the entire tower.... but it wont look the same....
I did some exploring back in my day and its not nearly the same inside as it is in the instance. I got outside the tower on multiple occasions and the instance actually stretches far beyond the bounds of the tower.
I suppose being that everyone zone is technically an instance you could "uninstance" it in a sense that it would be open for anyone to zone in.... but i have no idea how to do that, would just need to match the settings for the maps like "eastern kingdom"