I was thinking the smallest CFL city is Regina- pop. 236,000.
The Cdn. cities with a higher population are Saskatoon-295k, Windsor-329k, Victoria-367k, Oshawa-379k, Halifax-403k, St. Catherines-406k, London-494k, Kitchener(Cambridge/Waterloo)-523k, and Quebec City-800,000..
To me Halifax may not be a viable location as they are so far from the rest of CFL locations that they feel closer to the NFL New England Patriots. Quebec City on the other hand would be an excellent rival to Montreal, Victoria a rival to Vancouver, and of course any Ontario location would become a rival to TO and Hamilton.
Other than Saskatoon which would hurt the Roughriders in Regina, why can't the CFL expand into any of these other cities, if there was enough commercial interest to invest in a team using the Roughriders as a template.
Further to this, if there is no chance of expansion in Canada, I remember back in the late 1960's, the Eskies and the Lions played exhibition games, if my memory is correct, in Portland, Oregon for a couple of years in a row...population of Portland is over 600,000 and probably no chance of the NFL moving in to compete, I think they are close to the Cdn/Am border and could become a rival of the BC Lions much the same as Calgary/Edmonton because of their proximity to each other.
The only problem with expanding into the US is the Cdn ratio and I don't know if anything can be done about that