P: Exposure Selection Tool

Sure, it could work that way as well.

I'm still not sure what this tool would do. How is Lightroom supposed to know what the brightness of a certain object should be? Would it be an 18% grey picker?

Or maybe call it or have it be akin to a "spot meter" tool.

Maybe I need to clarify. An exposure selection tool could be used to set (or prioritze) the exposure based on a subject in the image as opposed to adjusting the Exposure slider back and forth until it "looks good" (same as when you use the White Balance sliders). 

In theory, color contrast affects the perceived brightness of those colors. So essentially, I'm thinking that an exposure selection tool would ignore a subject's color and would base the image's exposure on the brightness (or darkness) of the subject picked.

(refer to the samples below)

I don't think such a tool would really be that effective because there would always be some part of the image that would need tweaking. That's why we have the ability to add the different filters and brushes. The sensor in the camera doesn't have the ability to adapt like the eye and the brain does and compensate. I doubt I would use such a feature much at all.

I'm not sure how this tool is supposed to work. Sure, if I hover over the brightest pixels in the image, Lightroom could show me what the image would look like if those pixels are set to white. Same for the darkest pixels when set to black. But what is Lightroom supposed to do if I hover over any other part of the image? And how is this different from simply moving the Exposure slider yourself and see what that does?

I suppose the idea has some merit. However, with the versatility of the different brushes (sky replacement, radial filter, etc.) I question whether I would personally use such a feature very often. But that's just me thinking about my own workflow.

For instance: If you hover the picker over a highlight, it would show (+ click to set) you how much darker the image will be when you prioritize that particular highlight; and vice versa when you select a dark area.

I think it would be helpful as perceived brightness can be thrown off by certain colors and those surrounding them.

Do you mean like at different F stops? What would be the advantage of that over simply adjusting the exposure slider?