OK, you have a very nice camera with even very nice lenses, but there is still something missing to make the pictures you want - the accessories.
Accessories are a very important part of the equipment, because whatever you do: You don't need accessories to get the picture you want, but accessories can make it easier to get the picture you want.
There are many things you could buy. Some are useful, some don't. In the beginning it's probably hard to distinguish which you really need, which could be useful and which is nonsens.
For me, the most important thing is a good tripod with a good head.
When I bought my first tripod, I needed it for long exposure times
and experiments. I think many people think like that.
From my first days, I still have a Cullmann Titan (just the name,
it's not the material) tripod. I wouldn't buy it today, but for
the beginning it wasn't bad.
The big avantage is the flexibility of the Berlebach-System. You
can get your tripod in almost every configuration that you want.
You can hold it in your hand (wood, not metal), even when it's cold, it's very stable and well dampened, what you can easily see with tele lenses.
Which is better? Ballhead or a 3D-head?
First I bought a ballhead, because of the flexibility. You can
adjust it very fast and follow things with your camera.
If you want to adjust the camera very exactly, the ballhead is not the best solution, because while adjusting one way, it can easily happen that you also move another way. For this cases I use a 3D-head.
You can adjust every way on it's own and that makes it very
easy to adjust the picture exactly in the frame.
For those work I have the MA 410 gearhead from Manfrotto/Bogen. This is a 3D-head, but with a gear to adjust the head.With a gear head, you can control the frame very easily and very exactly. You simply turn the knob and the head moves in one direction. If you stop turning, there is nothing to fix - it fixes itself!
Whenever the tripod is to big or to heavy, I use the monopod RS8K from Monostat. The big benefit of this monopod is the very stable stand because of the patented foot. It also decreases the danger of unwanted turning of the camera.
For long time I had a shoulder bag. It had enough space for my equipment, but became uncomfortable when wearing it for longer than 30 minutes. Today I use it only for short time jobs or if I urgently need the quick access to my things. This is the biggest benefit of shoulder bags.
For long tours and hiking I use now a Lowepro NatureTrekker AW. It has also enough space for my equipment, but is much more easier to carry around. The waist belt takes at least 66% of the weight, if you adjust it correctly. The price you've got to pay is, that you have to take it off your shoulders when you want to put something in or out of it.
Filter and tools:
I only use the some useful filters, e.g. filters for adjusting
the color of the light (color-conversion filters) or filters for
manipulating the contrast in b/w-photography.
I don't like UV- and skylight filters at all. I didn't see any
impact of them, except reducing the contrast and reducing the
sharpness at focal lengths of more than 400mm.
For the camera I recommend a remote control or cable release. You will get sharper results, especially when using mirror lock-up. It also helps for long exposure times like 5 minutes or more, if you can lock the release button.