This has been posted elsewhere but it would be very nice to have a separate z-index property that I could apply on a node. For example as an independent setting that appears in the right-hand properties sidebar per node.
The specific scenario I ran into for this was I have an interactive Tooltip component that shows a details pane when you click on the ( ? ) icon. However, most of the time the pane appears beneath other layers because of their layer order (which is a product of auto-layout, etc.).
In my opinion it would also be fine if this z-index property was only active in prototype mode and had no affect on the actual Figma canvas.