Hi! I'm not a modeler either (but I'm working on it, slowly but surely), and like the idea. I don't think making scenery, like the Halleluja Mountaisn would be toe hard. IIRC, scenery objects can be made for FSX using Google Sketch Up, and I DL'd it and am going through the tutorials. I think the real problem with the floating mountains is going to be dressing the tops with trees, bioluminescent or otherwise.
Aside from the aircraft, the AMP suits would be interesting, along with mountain banshees and the great leonopteryx (With and without a Na'vi "mokto"), though I think they would be a strain on the average system due to the ammount of animation involved. But I think something usable could be done, if for no other reason, the technology is advancing fast enough to handle it on newer computers, and FSX supports "skinned" animations, from what I understand. It's simply a matter of graphics horse power.
Speaking of whih, Dire Horses would also be nice.
The only real problem I forsee is figuring out how to get Polyphemus to take up a large chunk of sky.