I love working with newbie villains. They always ask the questions that get you thinking, or at least show that they're thinking. Here's my favorite from Pyrepower.
"Rock and fire aren't on the periodic table. The four elements are magical, but if there's no magic, why can some people only control those things?"
The Aristotelian or "classical" elements are air, earth, fire, and water. Control of these elements is common enough that the Transhuman Capability Catalog has a whole section (TCC E) for people who manipulate them. So far there's no known instances of controlling "aether" or "akasha", probably because such things have no physical analogue. Notably, there are fewer wind-controllers in China and more people who control metal and wood, which are the classical elements familiar to Chinese.
During a stage transition, the body struggles to adapt to the trauma that's been inflicted, and is usually still being inflicted - for example, being in a burning car wreck. The powers that you get from that experience are often (but not always) powers that will save your life immediately and forestall future incidents of that type. Someone who is badly burned will get fire powers. Someone drowning or being crushed by pressure can get water powers. That sort of thing.
The really neat - and scary - thing is that your body seems to know what's causing the trauma, on a macro-level. It recognizes the problem from a holistic perspective and adapts. Most parabiologists believe in the Stubbs-Robur hypothesis, which basically says that people have the potential for a range of powers, and that the brain is constantly (and unconsciously) sending out activation signals to the body's cells for those powers. When the mitochondria in those cells reach an energy production threshold, the signals are allowed through and your new powers manifest. Most of these powers will concern the fundamental forces (electromagnetism, gravity, the strong and weak forces), and as soon as your brain feels the danger has passed, activation signal production ceases. Any limitations on the power from then on are based around your subconscious blocks, not around the mitochondria.
In layman's terms, you get as strong as you need to be to break out of danger, and no stronger. And you get only the power you need, not something more generic. This makes sense from a biological perspective, but it sucks for those of us who wouldn't mind a buffet of godlike abilities and are stuck with something specific. Since the classical elements are ingrained into your subconscious thanks to your education and upbringing, often your brain will accept the limited form of power that controls those elements.
Professor Pulsar described it to me like being an Olympic athlete. There's Olympic-caliber runners, jumpers, cyclists, shot-putters, archers, yadda yadda. But the specific things about their bodies that make them Olympic-caliber for their sport aren't the same. Look at the abs of a top-quality runner vs. a top-quality bodybuilder some time and you'll see what I mean. There's no "general athleticism" quality. In the same way, everyone's power development tends toward the specialized. The body doesn't know you'll want a big spectrum of powers for later, it's concerned with keeping you alive now, and it adapts accordingly.
I do know a guy who tried to work this system. He used hypnosis and conditioning to try and deeply convince himself that a certain type of power would be the most appropriate response to any sort of injury. I know what happened to him, but I won't talk about that here. Buy me a drink some time and I'll let you know how it turned out.