The main issues are overcrowding in the departure lounge, terminal road access during the morning peak and check-in space, the islands aren't fit for purpose anymore.
Construction on the shoreline check-in starts in September where they will knock down Joe's/M&S to Costa to put more check-in desks in time for next summer. Then Zone A will be removed and so on. So will look similar to Luton when finished. Once the new arrivals building is complete a second security will be built where the current international arivals is.
Charlie Cornish quoted recently saying he had expected more long-haul airlines and routes by now however he believes brexit has hampered these plans as the falling pound for outbound tourism and business uncertainty for inbound traffic. I expect if Jet2's EWR shopping flights are successful it might encourage scheduled carriers to service NYC. Although I do wonder what long haul airlines would come to STN?