Just a few things that would be nice imo.
Toggle Option: Show a dotted line around the eraser when you have it clicked, so you can see where your going with it.
Toggle Option: Disable the .jpg .png etc. quality check each time you save a file. Handy for those of us saving many images in a row.
Square Brush
Larger or adjustable thumbnail size.
*Additional ideas*
A preview window to show what the canvas will look like after resizing it could be helpful. I know I can do the math to figure it out, but it would speed things up when doing pictures in bulk.