I always thought Douglas was sort of the male version of Ann - it would make a great middle name since it "goes" with just about every fiirst name (although I know someone is going to prove me wrong now!). I think Allen and Marie are the other 'all purpose' middle names.
I am NOT, repeate NOT having another kid, but I think Douglas would be a good name if I did. My DSs are Andrew & Nathan, and I think Andrew, Nathan & Douglas work well together (Drew, Nate & Doug).