Aons are an interesting part of this book–perhaps my favorite of the world elements. If you think about the system I've set up, you'll realize some things. First, the Aons have to be older than the Aonic language. They're based directly off of the land. So, the lines that make up the characters aren't arbitrary. Perhaps the sounds associated with them are, but the meanings–at least in part–are inherent. The scene with Raoden explaining how the Aon for "Wood" includes circles matching the forests in the land of Arelon indicates that there is a relationship between the Aons and their meanings. In addition, each Aon produces a magical effect, which would have influenced its meaning.
The second interesting fact about the Aons is that only Elantrians can draw them. And Elantrians have to come from the lands near Arelon. Teoish people can be taken, but only if they're in Arelon at the time. Genetically, then, the Teos and the Arelenes must be linked–and evidence seems to indicate that the Arelenes lived in the land first, and the Teos crossed the sea to colonize their peninsula.
Only Elantrians can draw Aons in the air, so someone taken by the Shaod must have developed the writing system. That is part of what makes writing a noble art in Arelon–drawing the Aons would have been associated with Elantrians. Most likely, the early Elantrians (who probably didn't even have Elantris back then) would have had to learn the Aons by trial and error, finding what each one did, and associating its meaning and sound with its effect. The language didn't develop, but was instead "discovered."
There are likely Aons that haven't even been found yet.