New feature or functionality

Ability to open multiple images in reference image panel, zoom in and out of reference image, preview in greyscale, ability to rotate.

Enable RYB mode so that blending two or more colors, results in more natural looking colors. It would be nice to be able to mix Yellow and Blue and get green or Blue and Red for a purple.

It would be useful to have the possibility to merge the canvas with all the visible layers above it, into a new layer which should appear on top of them.

Ability to flip the quickly flip the document horizontally / vertically to check for mistakes and fine tune.

It would be helpful to have a shortcut key that will temporarily constrain the brush strokes to be perfectly straight when using the Brush Tool. Even though we have the Straight Line Strokes option in Painter property bar you have to click on this...

I'd like a function that allows me to use the transparency of a specific layer to mask other layers in my document.

Ability to warp, transform, distort an image or parts of an image with precision