I think shelves would be nice. Here are a few ideas for you:
http://deciture.com/shop/decor/wayfair/floating-u-shelf-set-3/ (could do multiple sets of these)

http://deciture.com/shop/furniture/world-market/emerson-shelving/

http://deciture.com/shop/furniture/world-market/espresso-charles-slanted-bookshelf/

Hope this helps you!

The entertainment center should be at least twice as wide so it's more proportional with the wall space.

LOVE your flooring. It adds so much contrast to your walls. Brilliant!! If it's art you are looking for, then we can help you customize an original just for you. It would bring in much needed color to your space. You can suggest the size, color and theme and it would be done to fit your budget. This way the painting becomes so much more personal.

Looks kinda dark? Is it on purpose?

Think about painting the wall to start off. It'll add color and create some depth. Here's a list of how-to pick the perfect color: http://www.mrcabinetcare.com/5-tips-pick-perfect-paint-color.html