Psych has their own APP problem, psych NPs are some of the highest paid APPs (besides CRNAs) and is one of the fastest growing APP fields. If they're not careful they'll find themselves in the same position as anesthesia.
Probably surgical sub-specialists, due to the relatively few physicians that are in each subspecialty (compared to EM, IM, Anesthesia, etc), and the length of training. I don't know a lot about derm, but from what I've heard talking to some friends in the field, derm is under attack from cosmetic clinics run by family/internal medicine doctors and APPs who advertise themselves as equivalent to dermatologists, and do acne scripts, injections, fillers, etc.
What I'm trying to say, is that unless you're willing to do 8+ years of miserable training and commit to a miserable surgical lifestyle, you're going to have to accept that you may have some midlevel encroachment, it's unfortunately a by-product of nursing administrators, the corporatization of medicine and the public's general distrust and dislike of physicians.