Tripod is a must have (if using the Astrotracer and Pixelshift capabilities) nice spacious and well padded backpack for all the lenses, at least one really fast memory card with sufficient capacity, two is better,
Battery grip (D-BG5, not necesarily original) is a good to have addition, I opted to add two sets of six-pack GP- Recyko rechargeable batteries so I can have enough battery capacity for every possible situation if needed.
Everything else is a matter of personal preference, depending on what you want to shoot. If you're into live macro (moving insects) a ring flash can be very useful. If you prefer to shoot wildlife and landscapes, it's pretty much useless.
And of course, lenses, lenses, and more lenses...