Inspect mode
Treat all frames without fills in the same manner like frames with fills – it is very critical for developers for better understanding of box models. Also for exporting vector assets with containers (icons/logos).
Outline mode
Right now it is useless piece of sh*t. No one even know that Figma has this feature. You can make it more useful by adding a very small amount of details. I believe it can be more profitable for developers than regular Inspect mode. And combination of them will be super powerful for design handoff. Please review my video.
Important: In outline mode also highlight text layers with rectangles even if they are not framed.