There are three aspects of any philatelic specialty to consider when you are looking around for a new area to collect-difficulty, cost and satisfaction. Specialties vary in difficulty and difficulty is not always relative to cost. Collecting the mint stamps of the United States is a moderately difficult undertaking if you plan to try to attain much completion in the pre-1900 period. Most US stamps are available mint with a little searching. But they are expensive. A good mint US collection, still with plenty of holes but pretty good, could easily run $100,000. That’s an expensive but not difficult specialty. Complete precancels of the United States are an impossibly difficult specialty even though except for a very few stamps nothing is more than a few cents a piece. (One of the world’s greatest collectors, with other collections worth millions, spent his life and resources try to complete US Bureau issue precancels and never could find scores of 20c stamps.)
Most collectors never consider how their specialty will play out over the years in terms of the amount of time they wish to spend, the amount of money they wish to expend and the degree of satisfaction that they hope to derive. For my money one of the easiest and most satisfying and affordable specialties is Confederate States stamps. The country can be completed for general issues for under $2000 (a modest sum for a major country) and there are only about fifteen different stamps, only one of which is hard to find. The only thing it lacks is difficulty, but you can always collect US precancels for that.