If you are interested in writing in Arnathia, here is where you'll find the things that are canon, per Kahuna. There is plenty of room to be creative and there are opportunities to write either sci-fi or fantasy, depending on the era of Arnathia you are writing about.
The one basic rule I ask all to follow is this:
Keep consistency. If something exists, pretending it doesn't won't work. And if you create something new, it needs to be consistent with the rest of the world. By all means create new things, but make sure they don't contradict the existing world.
If you write something that I really like, there is a significant likelihood it will end up here as canon.
As long as you don't change items that are canon, everything is good. Who knows? Maybe what you write can become canon as well!