These are all the secret sauce that very few affiliates will tell you. As OP said, you have to find your own unique niche.

If you write convertible copy, you can find an existing affiliate chain and try to use that to leverage your advantage. Otherwise, I recommend trying something that's out there, realize you won't be able to compete in your first niche (too much competition), then use that to learn.

Your first ROI might be 25%. Try again, find another niche, until you find the right one. This is classic explore/exploit game theory (every well publicized channel, e.g. the "Bingo Card Creator" of affiliate world) will be Red Queen'd to death.

note - not "great" copy, but copy that converts! (Or maybe you make landers that convert like crazy, or you have enough money to buy a ton of traffic and conversion optimize like crazy)