I think an oriental style rug that had some similar colors could work well. What are your remaining walls in the room going to be if this is only an accent wall? Will they be white or one of the colors from the paper? That would give a better idea of a direction for the curtains.

I would stick with simple accents. Let the wallpaper be the main event of the room. I think gray would look great with the wallpaper. Maybe nice dark gray curtains and pillows? Cream would be nice, too.

I agree! I would use some light blue cushions for the couch, with little or no pattern. For the curtains, I would go with something white and bright, as this wall and the light brown couch together with something else with matching colors would make the room too dark for my taste

I would take the colors in the wall paper and deepen it (or lighten) 2-4 shades and use that for accents. You don't want to match too much, and if you go with a color in the same family it'll blend nicely. I also don't see a problem with another print, perhaps spots or a stripe..as long as the sofa is solid.

I think a patterned rug is going to fight with that wallpaper. The paper definitely makes a statement on its own and should be the focal point of the room. For a rug, I'd go with a solid or a small, subtle tone-on-tone geometric, maybe ivory or a warm beige. I don't think a grey would look good with the brown sofa, unless the grey were very light.
I might even consider a sisal rug.
I'd opt for creamy window treatments in a nubby, textured fabric. If you could find some throw pillows with nice, soft stripes in tans, ivory, gray and pale peachy orange, that would be pretty. I think you need a little pattern in the room, but nothing that would become the focal point of the room. Let the accent wall do that.

I'm rather the opposite of Florida Joyce, but I get why s/he says what she does. Texture is fabulous, but don't run away from pattern merely because you already have a bold one in the wallpaper. A patterend rug may well balance the wallpaper. How comfortable are you with mixing patterns? I love to do it, but I'm one of those who keeps all my patterns in the same color scheme, but varies the scale and sheen, and my walls act as a solid color backdrop. I can't live with, say, this: http://littlegreennotebook.blogspot.com/2012/08/easy-envelope-pillows.html, but I love that even with so much going on, this vignette works.
I think this would be great with MCM and modern items, but be careful with mixing it and antiques--it might start to look a wee bit fuddy-duddy.
My advice? Take a swatch of your wallpaper, a cushion cover from your couch, your inspiration photo or one of the vases you own, a paint sample of your wall color, as well as anything else (small) that's important to have in that space, and hit fabric stores, department stores, antique malls, markets, Target, Home Goods, Ikea, whatever, and create a mini vignette of your own while you're shopping. Find what makes you comfortable--but don't settle for the norm--stretch a little.

I agree with a lot that has been said already -- especially with the use of gray color. I would suggest doing light matte gray curtains as well as a textured dark gray rug. The texture of the rug would take some emphasis from the pattern on the wall and help balance the room out. In terms of throw pillows, different shades of gray and blue (same tone as the blue on the wall, but a bit darker -- I believe someone else suggested this as well) with some sheen thrown in would work nicely. Make sure you provide a lot of light in the room, as a lot of these colors are a bit muted and need to be brought to life. I think this is where sheen on the throw pillows would really add to the room.

Not my colours at all but I think it might look nice with a lot of light neutral tones like, white, cream, sand et c. If you have a lot of white in different tones and materials to match your dark brown sofa and antique furniture (wooden) it could make for a harmonius and interesting room together with this wallpaper.

I'd bring a piece of wallpaper with me if you decide buying things with colour just to make sure it doesn't clash as the tones are a bit difficult to work with.

the wall will pop w/whites & light beiges, greens & yellows. it will be toned down w/ roses, blues, greys.

Mid-century-ish? What does that mean?