- Does this sound like a good idea?
Yes, I like the idea. As Minerjones stated there is already a Bitcointalk coin but I that one was unofficial. I'd love to see an official anniversary coin.
- What features would make the coin especially interesting?
Preferably a .999 silver coin. It would be cool if it came with a 1 BTC denomination (unloaded ofcourse).
- What's the best way to get a coin like this made?
How about organizing a community design contest? There are plenty of coin makers in the Collectibles section that can point you toward a good mint once the design is finalized.