There's a thing common to fantasy that irks me, but seems to be nearly unavoidable. It's the unrepeatable act.…stuff…
I dislike these as well. In my games they do not exist. However, that doesn’t mean there aren’t stories and such, but rather that the stories rarely tell everything. To use your own example, a story may tell of how Bess the Blacksmith acquired a great dragon's heart and used it to heat her forge and then forge a sword that was forever as hot as a dragon's fire, setting ablaze all it touches and granting its bearer immunity from all forms of fire.
However, it may have neglected to mention that she had been a master smith, that she had to make special preparations to prevent the forge it from literally going up in flames, that the sword she made represented an accumulation of work and experimentation with many failed attempts, that her final attempt taxed her so greatly she nearly died, and finally, in creating the blade so much magic and essence was drawn in from the heart itself that the forges magic was weakened to the point that a second such blade may have been impossible.
For the most part, it’d take some time to do between adventures, but if a PC wants to pursue such an activity in there off time, I let them. First they have to research the truths behind the legend and figure out how they are going to replicate the feat, they may even take an idea and decide to make something new or try to improve on the design, but they may also fail and need to try again several times before they achieve their goal depending on just what they are trying to do and how difficult it is.
Also if you are looking for direction I’d like to hear more about the martial arts. Being a martial artist I’m always interested in what others come up with, both the fluff and the mechanics.