A very nice collection! While not necessarily "a huge gaping hole" I think a 4" or 6" .357 mag revolver would make a nice addition to your collection. Since you don't have a Ruger in the bunch, a GP 100 would be my choice. I don't have very deep pockets and appreciate the value Ruger offers. A 686 S&W might go well with the 67 though. As already mentioned, a friend has the S&W model 17 Masterpiece and it is the sweetest shooting revolver I have ever handled. It is worth a look.
"Any fool can criticize, condemn and complain and most fools do." Benjamin Franklin