I collect anything that has good parts in it. The new Chima legend beasts are a good example. My kids love the new ball and socket joints, and we all love the new small slopes and teeth and the colors are fantastic. Since they are only $10, I've invested in many a copy of them since they are pretty much a parts gold mine.
I'm not sure where you are located so this may not be feasible, but I'd call the big Shop in LEGOLAND California before you buy. Last time I was there (about a year ago, but still long after the set was retired), they had them sitting on the shelf at regular retail price.
Nicely done! I see you have a torch on the stairs, but what about some torches on stands? This could be something along the front part of the plate to bring your eyes to the front and not just along the back walls. If you did it in black, it also might make some color contrast. You could also consider them lining the wall as you go up the stairs and this might create an interesting parallel line with the stairs.
If you totally wanted to go crazy, you could experiment with some of the impressively tiny new lights from Brick Stuff. They even have the ability to make the light flicker- how cool would that be!
And since your walls are high, you might consider adding in a tiny window on one or both sides. This would break up the space a bit and maybe offer a chance for some interesting details in the window muntins or frame.