Jumping in, the first thing I wanted to get a handle on was the overall world. Would all three games be on the same planet? What would it look like?
I decided to have two planets. One for the Dragon and Forgotten worlds, and one for Avalar. My reasoning is, Zoe remarks the world the eggs are taken to is 'on the other side of the world'. I have no reason to doubt this is true, as the dragon elder instantly knows of the place she describes.
I also believe that the Professor needing to use a crazy complex and dangerous portal to bring a dragon to Avalar, (rather than through balloon travel, or hole in the ground*), indicates dragons are in no way close to their world.
So, so far, this is what I think I'll be looking at when this project is finished:
Heraldic Dragons are scientifically important.
I have no idea what the lands look like at this stage, but I imagine the Forgotten Worlds would be bigger, as it has a far larger variety of climates.
Now, onto the rest of their solar system...
*Which when you think about it, should be his best bet, considering he's a mole and all.