Halifax.
I just blew your mind, didn't I?
The population of metropolitan Halifax is nearly 300,000. Weather is as mild as you can get in Canada east of Vancouver, allowing use of outdoor facilities throughout the North American season. On the East Coast, it is ideally located for USL-1's current teams and would introduce very few scheduling complications. In spite of its low population, the city has no summer sports higher than the CIS level to compete with a USL-1 team. It's a university town with plenty of the students and young professionals who traditionally form the lifeblood of a new football team in Canada. There's even a workable stadium in place: Huskies Stadium is artificial turf, seats 4,000 permanently and can be expanded to 11,000. Though also home to a CIS football team, the turf pitch ought to be able to handle the rigour.
The closest thing to a reason to turn down Halifax is population: 300,000 in the metropolitan area isn't that big. Halifax would be the third-smallest city in the USL Division One if it joined, behind only Charleston, South Carolina and Bayamón, Puerto Rico. But population has historically had only a small correlation with USL-1 success. Massive, immigrant-laden Miami can't draw flies to watch their side while Puerto Rico may be the most successful team in the league.
Yet Halifax gets no buzz for a soccer team at any level. If one talks about summer sports in Halifax one is talking about the CFL, in spite of the fact that there's no CFL-calibre stadium in the city and little prospect of building one. Halifax's history with the CFL and its history with high-level soccer are identical: they don't have one. It's peculiar, though emblematic of the state of the game in this country, that you can't swing a dead horse without hitting an article about CFL expansion while nobody considers the possibility that 4,000 university students might be drawn into watching twenty-odd soccer games a summer.
Remember, a USL-1 team, particularly one on the east coast which doesn't need to build a stadium, hasn't got huge overhead. Average attendance in USL-1 is under 5,000 fans a match. Halifax is a smaller city and not a traditional football market. But there's no reason it couldn't become the next big success story.