I've appraised nearly everything under the sun on the commercial side and I now specialize in healthcare: large medical office, specialty medical, and hospitals. Ultimately, it is the way to go, but it might be a long road to get there unless you start in a specialty group as a trainee and put in a lot of hours to gain the experience needed. Also, specializing is very difficult for most property types unless you are in a big firm. The caveat is the eminent domain guys, but they often have to wait 3, 6, 9, 12 months to get paid on a job.
Other commercial specialties are:
Gas Station/C-Store
Marinas
Multifamily
Hospitality
Senior Housing
Mineral Rights
Litigation (there are many sub facets of this and probably one of the more difficult and under served areas)
Large Industrial and associated properties (chem storage, distribution, heavy manufacturing, ports, etc.)
GSA work (could be anything under the sun)
There are others, but those are the ones that come to mind off the top of my head.