I just found libraw yesterday. Looks interesting, and I have no objection to pulling it in as an optional dependency. Pull requests accepted. :>
Freeimage looks like more than we'd need, and includes libraw. And actually, looks like it does a lot of the same things that Pillow does, wrt to various image formats.
Surprised you weren't aware of libraw (assuming DNG/RAW has been a requested feature for Pillow) -- but great it's on your radar now. FWIW libraw does an excellent job of handling RAW/DNGs -- as far as I know it's still the best tool for that (including various closed tools).
Freeimage looks like more than we'd need, and includes libraw. And actually, looks like it does a lot of the same things that Pillow does, wrt to various image formats.