Unfortunately most of the teams that are poor in attendance are also teams with a long history in MLB. Their fans, cities, and MLB itself would probably not take kindly to even the rumors of a move. Cleveland was mentioned, and Oakland and the White Sox are also near the bottom in attendance so far this season, as they were last season I believe. The A's haven't moved in decades and the White Sox have been in the same city for over a century. I just don't see either of them moving.
So it comes back to Tampa Bay. Frankly, I'm astonished that they haven't moved. There have to be a half-dozen better places for an MLB team in the USA or Canada. Montreal, Las Vegas, maybe even one of the places that have an NFL or NBA team but no MLB team? They could even stay in the South. New Orleans, Nashville, Memphis. What about a Carolina team? The Panthers and Hurricanes do okay, I'm sure their fans would love an MLB team to root for.