I'm not sure the relevance of whether Edinburgh is a "destination" or not has to do with its potential to become a transit hub. In a transit hub, it is neither the destination or the origin of the passengers - it is just a transit point. In theory anywhere can become a transit point.
However, to become a successful transit point, it helps if there is also a strong O/D demand. Notice I said origin and destination - it is irrelevant which direction they are travelling from. Manchester has far higher demand for transatlantic passengers than Edinburgh does, and with Norwegian also looking at moving into MAN, then this is far more likely to become a transit hub than EDI is, although it still falls a long way behind LGW.