I remember when the software Bryce had a neat feature called "zoom to selection".
You could render a scene, then use a bounding box to select an area, and then zoom in on that so that the selected area would fill the entire screen. I could make good use of something like that in Terragen because I often find it difficult to move objects manually, especially if they are at a distance. There are ways in Terragen to get closer to objects but that can often involve a lot of fiddling.
In the attached image, I manually moved the camera to the boulders; so it can be done but it is difficult to maintain the correct perspective, which of course changes as you get closer.
Interesting idea! Almost like a feature to "Zoom to crop region". Terrgen would try to come as close as it could to a freely defined cropped region, or maybe if there was a checkbox to "constrain crop region to aspect ratio".