Portland, OR, Montreal, and Nashville would be my top 3 cities for relocation. Tampa/St Pete is a horrendous baseball market. If it can't/won't support the Rays, it doesn't deserve a team. The Rays are one of the smartest franchises in all of baseball. If MLB puts that in a market that actually supports it, it would be the best thing for baseball and the franchise.
If relocating Tampa gains steam, I would expect that other cities if they can find some deep pocketed individuals to collaborate will jump in as MLB is the slowest 'moving' league when it comes to expansion and relocation (versus the NHL and MLS for example). Plus, a baseball stadium creates the most people traffic simply based simply on the number of games played each year and that traffic yields a lot of additional development opportunities
Potential North American MLB Cities:
Base) Tampa - #24 Metro 2.975 Million, #11 TV Market, #65 Avg Inc $21,800
Fun) Hartford/New Haven - >#50 Metro 2.063 Million, #33 TV Market, #8 Avg Inc $34,300
1) Austin - #46 Metro 2.000 Million, #40 TV Market, #21 Avg Inc $24,500
2) Charlotte - #32 Metro 2.426 Million, #23 TV Market, #31 Avg Inc $23,400
3) Indianapolis - #47 Metro 1.988 Million, #28 TV Market, #34 Avg Inc $23,200
4) Las Vegas - #42 Metro 2.087 Million, #39 TV Market, #84 Avg Inc $21,200
5) Monterrey - #17 Metro #4.475 Million, #2 TV market (Mexico), N/A Avg Inc
6) Montreal - #19 Metro 4.045 Million, #2 TV Market (Canada), N/A Avg Inc
7) Nashville - >#50 Metro 1.903 Million, #27 TV Market, #26 Avg Inc $24,000
8) Orlando - #34 Metro 2.387 Million, #18 TV Market, #82 Avg Inc $21,200
9) Portland OR - #33 Metro 2.389 Million, #22 TV Market, #10 Avg Inc $31,400
10) San Antonio - #35 Metro 2.389 Million, #31 TV Market, #182 $18,500
Montreal checks-off a lot of critical areas; but, it is in 'Canada' and MLB may not want to just have 1 team in the country's 3rd largest state by population. In an ideal world, Miami and Tampa are merged and move to Orlando opening an expansion slot one of Montreal, Austin, and Portland, which in my mind are the top 3. Vegas is too small and has low income levels on top of an obvious very expensive indoor stadium due to the heat (Vegas's AAA team has seen an attendance boost due to a new stadium built in the Sumerlin suburb in 2019). Charlotte and Nashville are not quiet there yet population wise. Indianapolis is in the Midwest, which is saturated with teams and has a declining population. San Antonio has an income issue. Monterrey was talked about years ago to expand the MLB market; but, moving into Mexico right now is just not feasible for a range of reasons.