A "powers" approach to "class" design for fantasy RPGs?

I just hit on this on the ride home from work and haven't had time to properly sort it out in my head, so I apologize because this is kind of going to be all over the place.

When I was in high school, one of my favorite things to do was theorycraft 3.5 characters. Just poring over the ridiculous number of splatbooks I'd collected looking for cool class abilities or new feats or what have you and thinking about how to synergize them and how you'd justify them in the world the character existed in was almost as much of a draw for me as actually playing. Hell, I probably made 10x as many toons as I actually played. Unfortunately, I feel like a lot of the narrative-style games, while good in their own right and certainly containing a lot of inspiration, don't really hit on that "mapping out your character" feeling.

The downside of such a system is that mapping out a character and feeling like you've got loads of options is that... well, someone has to come up with those options, so design is rather front-loaded.

As an example, I was thinking about a group in my world that are essentially were-turtles, and their warrior caste dual wield shields, both offensively and defensively, in addition to the shell on their backs. Nearby is a group of fire-aligned fey who can control and shape flames. So I was just thinking about a hypothetical character belonging to the first group who encounters the second, and perhaps wants to convince them to teach him some secrets. So in like 3.5, you'd need the turtle warrior class, the fire-fey class, and then you'd have to make sure multiclassing between the two isn't busted, or even potentially come up with a third class if this guy was to establish his own style or something, right? That's a lot of work.

Then I started thinking about supers-games, and how they have a list of powers, and then toons select from this list as they grow. Turtle warriors is a pretty narrowly defined concept, but "Fire" as a power is broad and can be reused for characters who focus on different aspects of the power. So instead of having to create all these classes, why not define the classes in relation to their "powers"? Maybe the turtle warriors have a Major power of "Defensive" and a minor of "Martial", the fey have a Major of "Fire" and idk maybe a minor of "Flight", and the third school has a major power of "Fire" and a minor of "Defensive", with the flavor coming from the turtle associations?

I feel like defining a power is a lot quicker than writing a whole class, and once the power is balanced it's theoretically balanced for any "class" that uses it. Is this a feasible approach to designing classes in a fantasy RPG? It seems to appeal to both the theorycrafting aspect (selecting powers, aspects of each power, etc) and seems like it condenses the workload significantly. Am I missing something obvious here? Does a system exist that operates like this?

Again I apologize for the rambling and verbose explanation but I wanted to commit it to writing before I lost it. I appreciate the time you took to read this and look forward to your thoughts.