The most obvious possible future dog is one that fills the role of the domestic dog's ancestor, the wolf. Feral dogs and even domestic dogs (especially those raised for hunting, such as beagles) show many qualities of social predators, including the pyramid of dominance found in wolves. Thus, it is likely these large pack-hunters would live in groups (mostly related individuals in a pack) led by an dominant male-female pair. These dogs have the best chance to evolve in North America and in Great Britain where wolves are largely absent or are endangered. There would only be limited competition from solitary carnivores like foxes and cats.
In northern Europe, the British Isles, North America, and parts of Asia where there are no major land scavengers (like hyenas and jackals), there are a number of openings for dogs to evolve into hyena-like predator-scavengers. Such canines would most likely be huge, muscular beasts with short, robust jaws designed to crush bones and rip flesh. Such Cujo-like monsters could be either solitary or pack hunters; individuals in packs would likely have higher pup survival rates, longer lifespans, and greater chances of catching prey. Certain insular environments could also produce miniature bone-crushing dogs, having evolved to snap the necks of island fowl and other creatures.
Perhaps a more unusual dog descendant would be a canine adapted to a semiaquatic lifestyle. Such a species could evolve from breeds like the retrievers and Newfoundland dogs, known for their webbed feet and water-resistant coats. They could range from medium to large-sized mammals specializing in preying on birds and fish, as well as other aquatic fauna. Long maws would aid in capturing such prey items, while long, thick legs would allow them to wade through currents to reach areas where such animals are prevalent. Although there may be lots of competition from aquatic birds and mammals like otters and seals, eventually such dogs could evolve into short-legged swimming forms resembling the prehistoric Ambulocetus.
Chihuahuas from hell
One of the scarier ideas for future dog species is that of tiny, ferocious dogs that live in huge packs on isolated islands and forests. This concept resembles the modern bush dog (Speothos venaticus). They live in huge packs of at least a dozen individuals, which allows them to take down relatively large prey. It is possible that they also become burrowers, perhaps with a population branching out and becoming subterranean cave-dogs, hunting underground rodents and possible specialized armadillos. The concept of these toy-terrors is open to all sorts of fantastic and strange modifications.
*All of the above concepts are open to modification and reuse for future world and alternative world projects.