Difficult to say without pictures but a subtle sophisticated stripe might work - either a gray or even a white/mica (especially great if you have stainless steel appliances). You could also use a matching or coordinating fabric for seat cushions and window treatments in the kitchen to tie it together. Or you could add a color for more "wow". You may find these pages helpful - one is "How to choose and use color in your home" and the other is about another option you could consider -- using fabric as a wallcovering (which is easier than you'd think, provides more options, and is as permanent -- or not -- as you'd like.) Also see #5 on this page for examples of using color and pattern to tie open spaces together.

It's really too hard to recommend...bring home lots of wallpaper samples that have various styles to help you decide. Look at lots of pictures on Houzz or in magazines, to get a sense of your style, etc. Wallpaper can be quite costly and it's best you narrow it down yourself and based on your rooms. Also consider a focal wall instead, which can sometimes be enough of a statement.