The problem with Halifax is the lack of a hinterland. It's stuck in the middle of a penninsula with no good land route to the st. lawrence valley and to mainland canada in general. Thus the port never really got developed as much as montreal or quebec city.
Really the best thing that halifax has going foe it geographically is its harbour and defensible location so we need either one of two things:
1. Have much worse Canada/US relations requiring a much larger naval base at Halifax and boosting industry through military demand.
2. Have Halifax be part of the same country as New England, allowing the port of Halifax to serve as a crossroads of trade between Europe, New England, and the St. Lawrence Valley... Although I can also sea Halifax losing a lot of trade to Boston in this scenario so maybe we need Halifax to be part of the same country as Maine and New Hampshire but not Massachusetts???