I'd like to see:
Repeatable texture as grain to a layer.
Ability to open a read-only pre-saved document as startup template on load that on save creates a new document.
A ribbon brush like in Clip Studio Paint so we can use real sampled ink brushes and strokes.
Brush shape format where Black is Primary and White is Secondary, but Hues in the brush shape are used as tones that the colour gradient of the brush will target. This way we can recolour sampled brush strokes with a hue/gradient shift.
Gradient map that has RGB separation to reduce loss of colour dynamics.
Perspectival repetition (like a mirror behaviour).
Gamut wheel with rotatable add/multiply gradients appliable to give realistic colour.
Alpha sampling from the layer (sending to say the brush opacity).
Ability to use a calculator-like math in the Gradients and Swatches between the primary and secondary colour and/or specifically selected colours so we can specify our own unique gradient rule.
Make it so the Hue slider can take a custom gradient/image instead so we can make a brush jitter from say an image (with alpha as well if PNG if possible).
Ability to randomise layer colour in Layers panel when a new layer is made, so we can more intuitively remember colours by their colour rather than being distracted trying to work out which is which by reading their names.
Panel that allows us to add brushes to as links to the actual brush panel and have this panel save with the document, so we can choose to remember what brushes we used for the document.
Panel that allows you to leave notes with B/I/U highlighting (richtext) that saves with the document, allowing easy reminders of what to do next in the document.
Ability to erase/recolour specific colours / colour range using any brush (so like a feature of the brush Eraser mode where everything but the colour range is erased/recoloured).
Specialised layers that on export do not show in the output saved PNG/JPEG, e.g. written comments, guides, basically helper/reference layers. And easy ability to toggle all of them being visible/hidden globally in the canvas view.
Ability to leave reference images around the canvas or drag them over without interfering with the layers. Like a built in PureRef that can save with the document and/or saved as a library of link/embedded images.
A ghosting tool, so that when we "ghost" / rehearse a line when using light/no pressure on the tablet, we see a temporary lighter line that then dissolves. This will help people make straighter lines and curves without having to undo them as much. Some artists have this in their eyes naturally due to afterimage disorders, so it is not a cheat to art, it is giving people abilities that they may not have normally. It may train people to draw better and image the ghosting lines.