It's difficult to predict specifically which specialties will have heavier caseload or higher patient influx, since it is dependent on location and population dynamics, as well as on the number of physicians entering each field. For some subspecialties, like CT surg, it is possible, but for comparable surgical subspecialties technology and medication advances, not to mention shifting patient behavior, makes it hard to be entirely accurate.
In general, our population is aging. Geriatrics and associated specialties will experience a generally higher patient load. However, does this mean pediatrics will have have fewer patients per pediatrician? It depends on the practice and the number of pediatricians. Generally speaking though one can say that surgical diseases of aging, such as colorectal or surg onc, will experience an increase in patient volume, but if you are a colorectal surgeon in a rural area with a sparse population your caseload will be light.
In terms of surgical fields, there will be fewer general surgeons, but more and more cases are being done by subspecialists. More and more vascular surgeons are feeling the pinch from interventional radiology and even from thoracic surgeons in some areas and regards some procedures. Bariatric surgery is a booming field, and with increasing rates of obesity may continue to be a burgeoning field, but all it takes for that to change is one effective pill.
In other words, your question is pretty complicated and defies a simple answer. the best thing to do is to find the type of work you enjoy, in broad terms (medicine vs. surgery, academic vs clinical, shiftwork vs. long follow up, rural vs. urban, etc.), and then specify from there. Skilled clinicians will always be in demand.
Just my 2 cents.
