Thanks for the suggestions, jbgibson.

One problem with the rivers is that I'm not entirely sure how to depict them such that they're both similar in style to the rest of the map and able to show realistically-scaled meanderings. I'm considering reducing them to 2 pixels wide (1 for the tributaries) with the blackish-brown coastline color. This would have the side benefit of eliminating those unsightly squared-off river heads and allowing for a higher density of rivers since they'd be less obtrusive. If I change the way I draw rivers, I might also gain some more leeway in extending rivers up into the mountains and the large forested basin as you suggest.

The darker green is indeed intended to be forests. As you seem to have perceptively detected however, the extent of this forested region was in fact generated from an altitude map. However, I decided to discard much of that altitude information which is why they instead follow some other (merely hinted at) elevation levels. This was kind of a hackish way to get out of having to hand-draw the forest borders.

In the source altitude map, all of the circular lakes (and dips, and forest rings) are actually impact craters. Your mention of Olympus Mons is appropriate, since the source material is a topographical map of Mars. I didn't want to draw undue attention to the many craters, which is one reason I discarded some of the altitude information.

As far as the mountains go, I've changed them once since the last version I posted and am now considering changing them again after having read your post. I don't have a good way of blending the mountains with the forest considering my primitive version of Photoshop and likewise primitive Photoshop skills, so it will have to be left to the imagination that some areas of forest are rugged and low-lying mountains are lightly forested. The shape of the mountains are in large part determined by the few Photoshop tools at my disposal, and their shapes within the mountain "zone" have no relation to the altitude source material.

Honestly, I have no idea what's beyond the edge of the map to the southwest. If I'd have to say something, it would probably be the roots of another massive mountain range. I'm fine with the 10% of the map closest to the edges being a little less polished than the middle, since the middle is considerably more important.

I'd say the main things I'm going to focus on after reading your post are: narrower and simpler river lines which allow more intricate river meanderings, more complex mountain ranges with smaller individual mountains, and better drainage of all areas of the map.