I have made a couple out of the Cake Mix doctor books and it seems whenever you add sour cream, yogurt, instant pudding mix, an extra egg, etc it makes it more dense.
I think sour cream does the best.
On the Cake Mix Dctr website they have a recipe for a "not so plain" vanilla cake, and it uses vanilla yogurt, an extra egg and vanilla instant pudding...and this cake is the moistest most dense cake I've ever had. The top of the cake literally looked wet from moisture (after it cooled) before it was frosted. It was out of this world!