The most logical choice would be southern Ontario, in the Kitchner-Waterloo area. The population growth is off the charts, and with Leafs tickets an overpriced pipe dream, a second team would make every sense.
In the short run they would probably have to play at Copps, and then move into a purpose-built facility in K-W.
My point of contention is that there are more teams in the NY/NJ area (three) and the state of California (three) than in the province of Ontario (two).
Winnipeg and Quebec City are not sustainable (Winnipeg being a romantic choice though), for Quebec City the tax situation is a huge non-starter.
Adding a 7th Canadian team would also, in theory at least, make the next round of Canadian rights fee bidding interesting. Would Global/CTV try to come in and bid for the CBC package given a second GTA franchise, and what of the cable package?
The wild card is that the Canadian teams have had a huge boost with the exchange rate, which was murder several years ago. Obviously if we're back to 62 cent CDN this is a nonstarter.
The corollary is that the impact in the US would be negligible, at best as the big-market US teams are not relocation targets. Plus, the free-to-air broadcaster pays zero upfront rights fees (NBC).
It's time for the NHL to realize that its "Southern Strategy" has been, at best, a mixed blessing. Sure, teams like Dallas, San Jose, and (to a limited extent) Anaheim could be called successes, Atlanta, Florida, Nashville are all disasters (poor attendance, money losing, tv ratings are poor). Carolina and Tampa, despite winning Cups, have very fickle fan bases. Phoenix should be doing better. Minnesota and Denver aren't southern but made sense and both teams have done well.