Are there any mods that cover the whole landscape in thick, dense forest? Similar to the forest in the center of the map, west of the Imperial City.

There isn't anything quite that pervasive that I am aware of, but, if you are not familiar with them, may want to have a gander at the Unique Landscapes project.

=) I have all the UL mods, they're great.

What I'm looking for, if not a forest, is a wilderness that makes the game seem larger than it is, to escape the "enclosed in a bowl" feeling I get.

How about try http://tesnexus.com/downloads/file.php?id=25023 and http://tesnexus.com/downloads/file.php?id=27235 to add more landscapes to the game.