I had a extremely wild daydream with five men. So as I thought their was mentioned that all D's are longly related another. I think that there were five brothers named "Monkey", "Gol", "Portgas", "Marshell", and "Jaguer".
I thought that the will of D could have been the independent strengh of these brothers and may be awakened by one in the D's once in a long generation. The will may even be there fearless attitudes, strengh, and destiny.
The D. makes me think Destiny, Destruction. Luffy may be the living vessel of this amazing power.