I might be tempted by a very think, light battery pack that the MBA could sit on and which wouldn't add too much thickness and height. But I probably wouldn't use it much (just my personal use).
Overall, I think the accessories are fairly well covered already. I have a WD Passport 320GB drive - tiny, light, bus powered and gives me a total of 400GB with my Air for not much more weight and size. I also have a Verizon wireless EVDO card - so I can have internet at a decent speed virtually anywhere. I have the included VGA display adapter in case I want to project a presentation. Finally I have a sleeve from Waterfield bags to carry it around without adding much bulk or weight.
The only thing I'd really like to see is a modified waterfield with a larger, padded and zippered side pocket. That would be a perfect way to carry the laptop and the accessories above in the minimal amount of weight and size. Think of all the performance packed in that tiny 5 or so lbs! 2 processors at a decent speed, 2GB of RAM, 400GB hard drive, internet everywhere, decent battery life. All in something that weighs less than a regular macbook without the power adapter.
