Hmm, some recent SE bonuses have been DLC things. Dissidia 012 had DLC costumes like KH!Cloud and some other characters outfits, XIII-2's bonus from Gamestop was a DLC costume for Serah and if you had a Power Up Rewards card (the paid one) you got a DLC weapon. From best Buy they gave people a book, and Amazon had another weapon.
When BBS came out, I remember the preorder bonus was decals for your PSP. Re:CoM was postcards with artwork on it.
Some guesses to what DDD will get would be something like a bonus similar to Japan. They are getting that special AR card with a rare Dream Eater on it if they get a first print copy of the game. Or something like the "visual history of the KH series" postcards in the 10th Anniversary Box.
I'd say if there was an AR card, it'd be worth a preorder. If not, I'd just go ahead and go to Walmart xD
No problem! Not many people come to ask me questions so when they do I love talking xD