My idea would be a stack-able band. I don't wear a lot of jewelry, my wedding band (my engagement ring gets caught on too many things), a Tiffany heart necklace DH bought me 3 years ago and a simple pair of white gold hoops. I would like a band that I could wear with my wedding ring. Something like
this. My wedding band looks very similar so it would fit right in. This way you wouldn't have to worry about it being way out of her comfort zone. It's something she could wear daily.
I also really love
this one, but not something that could be worn w/ a wedding band, for me it would have to be a right hand ring.
Here one more I just found doing "stackable ruby ring" in a google search.
http://www.diamondharmony.com/products/14k_White_Gold_Prong_1_6_Carat_Diamond_Stackable_Ring-2477-17.html
It's a diamond, but just to give you some more ideas.
I would definitely go with simple based on your description of what you wife wears, but something with maybe a little detail. As some PPs suggested, I would also go to a local jeweler. They're much more likely to work with you to create exactly what you want and give you some great prices. My jeweler will give us discounts on everything we buy off the sticker price if they've recently had a sale or are going to be having a sale. Also, make sure the type of gold matches your wife's wedding band (yellow vs white gold). Personally, I don't like to mix gold colors.