I like the density of information, so I don't think you need to worry about the blank spots. It does feel like the map is sort of sagging, though. The amount of information near the bottom, the graticule's shape, and the arc of the central label all contribute to that feeling. The suggestion of a title could counteract that, particularly if you give it an upward arch to counter the downward arch of the grid.

For the forest, you could try giving it a tufted border, as can be seen on many of Mike Schley's overland maps. Or you could scatter some isometric tree icons through it. They don't have to be dense, particularly since a very large portion of this map is forested.