East Carolina in the SEC could become the ACC's worst nightmare. ECU has tremendous potential to become a football power for a number of reasons, some of which you cited: solid following with a passionate fan base ; 27,000 students; location within reasonable proximity to the Norfolk/Newport News metro area (1.7 million people); 50,000 seat stadium which could be easily expanded, etc. Plus, ECU is a football school located in a state known mostly for ACC basketball. I can only imagine what the SEC could do with ECU football in the unlikely event it brought the Pirates into the SEC.
The biggest thing working against ECU is their not being the flagship school of the state. UNC would be the one the SEC wants, but, if they turned them down, they'd take NCSU over ECU, and, it would not be in doubt. ECU's following in eastern NC is pretty strong, as you correctly point out. You are also correct about their location, relative to the Tidewater region. It is another huge positive.
But, the further west in NC that you go, the smaller their following gets. Also, the further south you go. By the time you reach Wilmington, ECU is a non-factor. They could develop that reach, if they continue to have the success they have now. But, a spot in the SEC is a pipedream for them.
I have a VHS tape of the 1990 Virginia Tech/Virginia football game, and during one of the time-outs, the proposed Big East Football Conference was being discussed. Syracuse, Pitt, Boston College, Miami, West Virginia, Rutgers, and Temple were set to join and the discussion was on who would likely become the eighth and final member. The candidates--Virginia Tech, Louisville, and East Carolina. Things worked out pretty well for the first two.