I've always dove in head first with the mechanics and then ended up with a full list of mechanics without any real
world to put them into... so I'm going to try the world-building part first since the mechanics are the part I enjoy most
There are 7 "Elements", each represented by a gem type and 5 crests, which represent party roles. Combining the crest and the gem imbues a character with the potential to learn related skills (so you literally craft your classes). This does lead to the interesting side effect of having 35 individual classes to create x.x
They arose by virtue of the soldiers imbuing their knowledge into the gemstones over generations. The crests just unlock which knowledge is learned. There were originally 7 kingdoms, 1 for each element, each with different fighting styles (so the "element" of the gemstone also represents the fighting styles of the region). Of course, now there's just 1 empire, having absorbed all 7 kingdoms.