>>>There's speculation that's why that they might setup another hub. However, it's travelers to/from Toronto that lose out from the lack of a direct connection in this case.<<<
If they expand beyond the current 20 planes and look to expanding into multiple hubs, I think the best approach would be to target routes which are currently poorly served by nonstop service to Toronto.
Probably the best candidates for hubs would be Ottawa and Thunder Bay (especially if they increased service to Thunder Bay to 6+ flights per day). Most of Canada can be reached from either of those. With frequent service to YTZ, the connection time doesn't need to be long.
From Ottawa they can serve Quebec and Eastern Canadian cities such as Halifax (as they already do), St. John's, St. John, Fredericton, Moncton, Quebec. Connecting through Ottawa will be easy and quick given the number of YOW-YTZ flights there are every day now.
From Thunder Bay they can serve western destinations such as Saskatoon, Calgary, Regina, Edmonton (perhaps later Edmonton City Centre if the rules there ever change) and perhaps some northern destinations like Yellowknife (direct or via Saskatoon/Winnipeg) and Iqaluit (and on to Nuuk?) which don't currently have good Toronto service.
Later they could add US destinations from Thunder Bay and Ottawa to support connecting service and look beyond supporting traffic to and from Toronto. As well, direct Thunder Bay - Ottawa flights would allow for east-west-north connections.
While I don't see Porter competing very much for Calgary - Toronto passengers, I think that there would be traffic for various portions of a Calgary-Saskatoon-Thunder Bay- Toronto flight, especially if there were connections from Thunder Bay besides Toronto (Chicago, Minneapolis, Iqualuit, Ottawa etc). The trick is to look for combinations not well served by Westjet and Air Canada.
EDIT: The key is for the hubs to be relatively non-busy airports that allow for quick turn around and as few delays as possible. The Ottawa stop from Toronto to Halifax isn't very long and doesn't really add too much to the total flight time. This allows Porter to compete with nonstop Halifax - Toronto flights. The ability to land downtown Toronto and a much quicker checkin process (you don't need to stand in an Air Canada line a thousand miles long) in Halifax make up for having to make a stop. Inexpensive is important too.