I do believe we will see something in the next year or two. There are just too many tour pros going the way of an "anchored" putter for it not to be an advantage. The argument that I've heard against TW's "no longer than the shortest club in the bag" is that back trouble requires some to have a slightly longer putter. If you cannot bend over without pain, should you be allowed to use a longer putter if it is not "anchored"? Raymond Floyd was one who used a longer putter, once his back wore out from a life of golf and "other entertainment". What about those who have been sold on the constant length iron system? If all my irons are the length of a 6-iron... even the wedges... I could carry a longer putter than someone using standard lenth clubs. Or, I could just learn to play with clubs that were 1" over standard and have a little longer putter.
They also have difficulty defining "anchoring". Langer used a regular length putter, but "anchored" it to his left wrist with his right hand.
I look for the USGA to come up with an artibrary maximum length, sorta like they did for the displacement size of drivers.