The idea should be to try to capture as much of the Western GTAH market as they can. It's unlikely that passengers coming from East of YYZ are going to choose YHM unless the price is much lower. Oakville, Burlington, Kitchener, Hamilton, etc would have an easier commute to YHM compared to YYZ.
YVR/YYC/YEG/YHZ are good initial choices for service. As mentioned above I'd hope for service to large us markets as well (like NYC, CHI, ATL, LA, MIA, WAS). I'm cautiously optimistic about this. After all WestJet started out at YHM initially and moved to YYZ as quickly as they could. YOW and YUL are notable absences though Porter provides ample service from YTZ/YYZ and I'm not sure how much demand there is for YHM-YOW/YUL service.
***EDIT***
Looks like Porter is scheduled to begin receiving 25 more E2 aircraft in 2025. So they may be waiting for more airframes to come in before adding additional flights. Also according to Plane Spotters they do have 4 E2's currently parked at YOW that can be deployed at YHM.