How can I decorate a big slanted wall?

You could get a nice looking tapestry to match your living room:

http://imagecache.allposters.com/images/pic/PUR/20...

You could always use something like stick on vinyl from a signwriter to make a picture or have words that are meaningful to you on the wall. I saw a great one in a house that had a soft green wall with huge white vinyl leaves from different varities of herb plants stuck on the wall. It was very restful.

Otherwise, what about a patterned wallpaper?

You could paint it black and use white paint to write in cursive writing, love poems. And you really don't have to use black , you could use like a soft suede color and write in white, or brown. As you may already know the colors you could come up with are endless. Not small writing like you do on paper, try it large as much as 4"in. Write more than one, it's your wall to be creative.

maybe a fire place and white carpeting (makes it feel warm)

a chandelier

you can put up a plasma screen tv (to take away from the slanted wall)

I think you should accent it, paint it as a accent wall, or paint stripes on it.