One possibility for definining how it could be, is taking inspiration from oriental martial arts where they differentiate between a -jutsu and a -do.
A -jutsu art is all about the techniques and carries no superior purpose (e.g. ethical) because the techniques *are* the purpose, while a -do art treats the techniques themselves as the mean to achieve a higher purpose to (put in blunt and naive terms) become a better self. So you have jujutsu vs judo, kenjutsu vs kendo, karatejutsu vs karatedo... The sword techniques taught in kenjutsu and kendo might be practically identical, but kenjustu sees them as the purpose (i.e. learn how to fight) while kendo sees them as the mean (e.g. learn how to improve). This is very simplistic, but just as an idea...
So one way to conceive the Fighter class could be (without any oriental flavor), someone who studies a "jutsu".
Just to point out: do vs jutsu it's just a matter of fashion. Old martial arts are named "jutsu". In the XIX century, it becomes a "new fashion" to build up new martial arts that simply removed the "martial" part of it. To use the name of a thread in this very same forum, "jutsu" is "combat as war", and "do" is "combat as a sport" Kenjutsu was, since XV century, the art to fight with a real sword against another real fighter that was trying to kill you. Kendo was the art to fight with a wooden bamboo shinai, which was developed MUCH later.
Same goes with Jujitsu, which is very ancient way to fight, including killing moves, compared to judo, which is a sport from late XIX century, Jigoro Kano invented it in 1882.
Bassically, most "justsu" martial arts become "do" sports in XIX, de-emphasizing the martial part of it to adapt them to a more modern idea. Iaijutsu became Iaido, Jujutsu left behind strikes and most killing moves and became Judo, Kenjutsu left behind real katanas and become Kendo. Kobujutsu left behind real weapons and became Kobudo. And so on.
There is not a single ancient martial art (by "ancient" I mean X to XVII centuries) that is named "do", but that has nothing to do with "being a way". Ninjas were "a way" more than any other martial artist, by your own definition, and they practice "ninjutsu", not "nin-do". Tai-jutsu, Dakentai-jutsu, iaijutsu, kenjutsu, all of them are from XV century. Kendo, Kobudo, Karatedo, Aikido, etc, all of them are from XIX century and beyond, and all of them are "combat is a sport" compared to the previous "fight for your life, winning is surviving because this is war" mentality