I'm also guessing work will begin on the new arrivals hall at the end of the summer season, less disruption airside as possible while the summer flights are still in progress. If it's going to be so quiet this winter then it makes common sense to wait a short while & have it all ready for spring the ramp-up next year.
The old "Nat's" building will be demolished, but I can't see it being used for any further extension to the departures terminal, possibly only a tweek to the hold baggage make-up area is required to give it room for future growth.
Hopefully the Nat's footprint is earmarked for some sort of road development to ease the vehicle entry/exit side of things to the airport, a good sized junction with lights or even a large roundabout would help keep traffic flowing at busy periods.
I think there was also a future plan for a car park along the perimeter fence line, maybe it would start from the reclaimed Nat's area??