Maybe you can use a bold color, meaning that you should chose a wallpaper that is a color like brown, just brown. It's more traditional, but the real question you have to ask yourself is that do you want your wallpaper formal or homey?

The carpet is more traditional and formal in color and pattern, so I would choose a larger scale wallpaper with a bit of punch that speaks more transitional (like large quatrefoil) or a grass cloth textured wallpaper in a fresh color adding texture. Larger circular rug too.

I would try paint before wallpaper. You already have pattern from your carpet, I don't think that this is a situation where pattern with pattern works, not for a transitional look that you seek.

Also, I think patterned wallpaper looks best in spaces with classic proportions. Your upstairs hallway is on the narrow side, and from your picture of the upstairs hallway, a wallpapered kneewall facing the wallpapered perimeter wall will make the space feel closed in.

I like kovar's suggestions - start with paint and accessories, new interior doors, trim and hardware if budget allows. Go to wallpaper only at the end if after everything else, you really feel like your space needs that extra layer.

Great stain choice for your oak floor, by the way!

Check etsy for a wallpaper stencil. It would be fun and it's just paint.

How about a Thibaut?