You could always get those cheap rubbermaid shelves that are designed to go under sinks. They're stackable if there's enough room for more than one. Then you could stack things like out-of-season clothes/sweaters, t-shirts, shoe boxes. There are also little drawer systems that are small enough to go under sinks that might work for smaller items.

I have an Ikea wardrobe, too. I use the top shelf for out of season clothes, so right now I have my bulky sweaters and sweatshirts. I feel like it's still not used though. Mine is super tall and I'm pretty short so I don't use the top shelf mostly because I can't reach it. So that's not very helpful, but that's what I do.

Add a layer of shoe boxes and get those shoes up there. Get the kind that are little drawers. Then you can put other stuff above the shoes.

You could always move up the top shelf by say half and then raise up the clothes bar so that an extra space is opened up under the hanging clothes. It's probably also a matter of personal taste though.

1) Add a second shelf up there and use it for out of season or seldom used clothes in a container.

Or, use it as I do

2) two suitcases fit perfectly up there, on their sides, laid along the longer side. And I fill them with stuff like hiking boots, travel supplies, bulky fleece stuff (out of season) sleeping bags etc.

If you want to move the rod up, get NEW brackets and mount them higher.

You can then move the shelf up, or put it under the hanging clothes.

When you have a big, deep space that's not accessible, put big, thick stuff that you don't need to reach every day. Could you use that space for blankets, suitcases, big tub of out of season clothes (if you live where you need winter clothes)? Or humidifier, fans, other occasionally used machines. I have a big box full of odd size shipping boxes in a spot like that.

I have a similar wardrobe, and I moved the hang bar as high as it would go and put the shelf below the bottoms of the hanging clothes. Combined with the bottom of the wardrobe, I now have two shelves down there for t-shirts and sweaters.

When I put my ikea pax wardrobes together, I purposely didn't put the rod too high up, as it hurts my shoulder and arm to have to reach hangers with suits on them up that high, as I'm not all that tall. And I didn't even buy the tallest ones! (I currently live in a place with the top rods installed in the closets too high for me, as there are double height rods.) I much prefer a rod placed at a comfortable height, even when it leaves harder to reach shelves above. The only way to reach them is to have a stepladder nearby. Even then, it is difficult to reach all the way into the back part of the space. So if you need to use them for stuff you need to access frequently, I would suggest keeping stuff you don't need frequently in the back of the shelves, stuff not too heavy, like blankets, quilts, pillows, and use the front part of the shelf for anything like sweaters, shoe boxes etc. And get a great folding lightweight stapladder, the kind with a rail to hold onto above the top shelf, and stash it next to the wardrobe.

To make better use of the height, you could use kitchen shelf organizers to give yourself another tier to put things on. There are even some that you can stack and offset slightly, which could take advantage of the shelf depth as well (e.g. http://tinyurl.com/d5btdno).

I have 2 Pax wardrobes that are 7 ft high. I'm 5'2". I bought two of those big black zipper boxes that fit the entire width and depth of the wardrobe. I placed that top shelf at the height of the zipper box so that it fit perfectly in that spot. I use it for linens, too small, too big, out of season, shoes, everything. And I can manage to get a fully loaded zipper box up on that shelf alone so it can't be that difficult. I also don't have the doors on my Pax so it gives it a clean look on that shelf plus the zipper box keeps everything nicely protected from dust, etc.

The depth problem can be solved with one of those roll-out cabinet shelves/drawers from Container Store, etc. Really easy to install and very sturdy. I got one for a deep kitchen cabinet where pots and pans hadn't seen the light of day in years.

To maximize the height space, top the above roll-out drawer contraption with an adjustable tiered shelf or two.

One pull and voila! All items in full view and access...