Both Bucharest and Warsaw have been served by easyJet from Gatwick before. It seems neither worked out for them, so maybe they will try again, maybe not.
Vilnius is interesting and i believe it is one of the most likely of the Aer Lingus routes to be taken up. Another interesting airport is Eindhoven, however if anybody is to take it up maybe it would be Transavia?
I personally think that the future expansion will be more of the mid-long routes we have seen popping up recently. These have been doing very well for the airline so i wouldn't be suprised.
Is the open skies agreement with Tunisia not come into effect this Summer too? easy have shown interest there in the past, so theres a possibility, another perhaps could be Cairo? Beirut?