MECHANICS SUGGESTIONS
If mechanics aren’t solid yet, I have some suggestions I can toss your way (I take rejection well, so fret not):
what if there were classes that acted solely as frameworks for how you spend your points leveling, but also didn’t restrict your selections.
Example: mage class would have a point discount for magic stuff, but martial endeavors and athletics related stuff would be more expensive. so you could pick Mage but dump all your points into fighting with swords, but it would likely be inefficient.
on the flip-side a Warrior class could be throwing all their points into magic stuff, even though it would be more efficient to go with martial stuff.
Am I way off base on your mechanics goal here?

An idea that came to me from a webcomic (that I discovered through someone's sig): what if powering your powers required an energy source, generally the environment, but nor necessarily. so you would need Heat energy to fuel your fire powers. and you could use your fire powers to set the things in your environment on fire to generate more heat. But maybe you level is the limit of how much you can utilize at one time, or even store semi-long-term.

Actually expanding on the idea, storing too much of your energy (assuming the energy is being stored or is internal to begin with) could be bad.
Let’s keep using heat as an example: store too much heat and you could catch on fire. Store too much cold and you could start to slow down and maybe even freeze.
So a mechanic for releasing your energy besides using your power could exist.

Even further though: what if there was a risk of your power controlling you instead of the other way around.