Arctic Lions: Yes, really

Wild lions (Panthera leo) are today almost entirely restricted to Africa, the exception being the small population of Asian lions (Panthera leo persica) in Gir National Park in India. This was not always the case however, lions were in fact once found in every continent but Australia and South America. There were many lions in North America, including Canada. The American lion (Panthera leo atrox) was found in both arctic/taiga environments and plains/steppe environments.  

There is a lot known from fossils and DNA evidence about American lions. For example, it is known that they were significantly larger than modern African and Asian lions. This was presumably an adaptation to hunt larger North American herbivores such as bison and camels. From genetic evidence we know that it was more closely related to Asian and cave lions (Panthera leo spelaea) than to African lions, and so probably shared some characteristics with them. Asian lions have short manes compared to African lions, and cave paintings of cave lions seem to imply they did as well. From this it is supposed that American lions also had short manes. It is also presumed that like all modern lions, they had a pride structure with females being the hunters and males defending the pride from other males and dangerous creatures such as elephants and buffalo. It is not known exactly how their fur grew but presumably it must have been thick enough for them to withstand cold winters. 

Modern lions are not typically found in colder environments, but many captive ones are and have shown a surprising ability to withstand low temperatures. Many grow thick manes and pelts in response, with lions regularly being kept outside in the winter in places as cold as Scotland, Canada, and even Siberia. This adaptability is probably what allowed lions to have such a large range in the first place. It is thought that many of North America's large carnivores went extinct when many of the large herbivores were extirminated by human hunters. In the lion's case, this was the horses, the camels, and the bison. When the horses and camels were gone, and the bison few in number, the lions starved. Without the lions present the remaining bison could grow to the huge population size seen in history.  

If the lion's prey could be brought back to Canada, then supposedly the lion could be as well. The Pleistocene lions are considered subspecies of the lions still present today, so the differences are minimal. Substituting the American lion with lions from the old world should nicely fill the ecological role left behind by the American lion. The Asiatic lion from India might be the best choice due to it's closer relationship to the extinct lions, but really any lion subspecies would be acceptable, and African lions would certainly be more readily available. Lions could help to control native deer and bison populations as well as the numbers of species to be introduced later.

Picture comparing African and Asian lions (https://i.ytimg.com/vi/DAyaRfy8sPU/maxresdefault.jpg)