I think 4 daily flights between the Middle East carriers would be overkill and we'd see load factors below 50% for good portions of the year.
Sao Paulo
Seoul
Copenhagen
Lisbon (by TAP)
That would be the order in which I would like to see new services added. A non-stop flight to South America would round out Logan's network nicely. Europe is well covered with daily flights to all of the big hubs (London, Paris, Amsterdam, Frankfurt and Munich) plus flights to Dublin, Shannon, Madrid (seasonal), Rome, Zurich, etc. At this point Seoul is the only missing link in Asia.