Implement marquee-selection of the capture bitmap, so users can cut/copy/move parts of the image. Basic image editing would be a major improvement, and something most other capture programs have (PickPic, HyperSnap, etc).
Greenshot wasn't designed to be an image editor, there are applications which are extremely good in such things. We only want to make sure you can annotate your captures.
I would advice you to use the external command plugin so you can open your capture in Paint.NET, maybe we will improve the way back to Greenshot so you can edit in an external editor and than upload it with Greenshot.