Possibility to reposition the grid
Possibility to move the grid so the half-sheets are at the top rather than the bottom, for example if one design element shouldn't be cut in half.
Yes, it woul be nice to center images in a four-sheet 2x2 grid so that all pages have equally-sized images.
Daniel Hutton commented
You can do this somewhat by rotating your image 180 degrees before uploading to rasterbator. That or adding a white border to one side of the image before uploading.