The oldest modern breed standard that I can find is the 1879 Dachshund standard.
Most breed standards are limit the “correct” conformation for their respective dog breeds. However, if you look at the Dachshund standard (FCI), you’ll notice that there are three coat types and three sizes ( The “rabbit” and miniature sizes are combined in the Anglo-Saxon registries.) Dachshunds cannot be piebald or solid white, black, liver, or gray, but other than that, they come in virtually every color.
So the dachshund standard really didn’t limit this breed’s conformation. It merely circumscribed it. And within that circle, some diversity was allowed.
And long-haired dachshunds are different from short-hairs and wire-hairs, which are different from each other. A 25 pound long-hair is more of a spaniel, while an 8 pound wire-hair is going to be more of a terrier.
And yet these dogs are all one breed.
Now, interbreeding coat types isn’t done. If you cross a long-hair and wire-hair, you won’t get a coat type that breeds true. You’ll get an interesting coat, but not one that can be classified very easily.
Sizes are interbred, but because breeders want to be able to predict size in their litters, this is almost never done.
And while it’s true that the fancy has produced bizarrely short-legged and long-backed dachshunds, the original standard allowed for diversity. The current standard does allow for more diversity– much more than you’d get in a golden retriever or a Bedlington terrier standard.
So maybe if we really want to improve breed standards, we take look at the first one and consider how important diversity in type really is. After all, the reason why dachshunds come in three sizes is because of their expected quarry. A bigger dog will go after badgers. A mid-size one will go after foxes. And a little one will take on a rabbit.
And maybe allowing for a little more diversity will allow for a little healthier gene pool in all of our breeds.
One of the great things about my preferred breed (Belgians) is that there is no restriction in breeding and mixing among the different coat varieties – except for the US where they are considered different breeds. Mostly this is done by breeders of sport/working dogs. This has maintained a relatively good gene pool with a healthy diversity.