Finding relevant code, possibly during onboarding of new devs
That one is huge for me. So often coming into a new codebase (especially in the case where some of the libraries or underlying frameworks are new to me) I just want to know "How the F was that value populated??"
I can already imagine the dev tool improvements that could come from something like this.
Being able to highlight a part of the dom and click through the stack of where it came from would be awesome! And it would work with just about any framework out there!