The problem is that downtown was gutted by the creation of the Halifax Shopping Centre and Bayer's Lake. Downtown shouldn't need to have a big population to remain the centre and heart of the city. Most British cities don't have a huge population living right in the centre (they're generally all retail and commerce), but the centres have such great facilities that people want to head into them from the suburbs at the weekend; it's a 'day out'. People come in to shop, have lunch somewhere nice with their friends, visit the library, take in a movie, sit in a nice public square and chill out people-watching, check out an exhibition, have some drinks at a bar; just taking in the lively atmosphere. You can do them all within walking distance so it's a great place to come. There's no need to drive a car from each individual place to place and do things one by one.
I don't really feel that you need a lot of nearby housing to support a great downtown. How much housing is there within walking distance of Bayer's lake? The facilities draw people there.
If you build it, they will come
Here's hoping there's space to squeeze in some decent shopping space in the remaining vacant downtown lots. Oh, and here's hoping the council lower the rents for businesses downtown!