In my opinion there's zero chance SJU is ever invited to join the ACC, or any other non-football school for that matter.
The ACC doesn't need to add a school - having 14 in football and 15 in basketball is fine. Adding an additional basketball school will only add one more hand that needs to be fed from the conference's TV deal. Why split the pot with more partners? Unless adding that team provides more value than an additional hand in the pot would cost. But no basketball program available provides that kind of monetary value. Basketball games, even great ones, don't move tv ratings.
Plus, having an odd number of basketball teams doesn't pose the same problem an uneven football schedule does. They're going to have to move to a format like the Big East has used, and teams won't be playing everyone twice as it is. The schedule will be uneven whether they have 15 or 16.
I know the sporting news and others have mentioned rumors, but this doesn't make a bit of business sense to me, so I can't see it happening. Ever.