The National Stadium ship has long since sailed...and before it left the harbour (no pun intended), there were plenty of other tempting sites (North Foreshore, Titanic, Scirrocco, Maze...)
3i will do two things...attempt to develop what space is available around the site, and try to grow the airport itself. Neither is anything to be concerned about.
A runway extension remains a cost-benefit nightmare...ain't going to happen.