To be a modern and sane C++ that C++ could have been, (rather than a complex collection of tacked on languages that C++ is), with modules instead of the the mess of C++'s headers, with instant compilation times that does not need a compilation server farm.

One good case for it that I see is a viable basis for cross-platform desktop apps. Today, cross-platform desktop GUI apps are either just a snapshot of the website contained inside Electron, or a C/C++ code base with manual memory management. D can serve as a nice middle ground in that space.