This is just imaginary thread since government cannot give PIA $1 billion. Perhaps when gov't sell 26% stake and the new company injects similar amount.
My choices for fleet are:
ATR-42 and ATR-72
CS-100
A320
A350-800
B777-200ER
B777-300ER
The reason I picked A350-800 over B787 and A350-900/1000 is because this plane has long range. It can serve places like Sydney, South Africa, Western North American, Latin America and so forth, which a330-900neo cannot. The 3 class seat is 270, which is between a320 150 seats and b777-200er 330 seats. Plus maybe PIA get 777-8x in future, which is 353 seat. Also a350-800 sell only 18, so PIA can get good discount. Assuming 50-60% list price discount, which is 130m~, that's about 8 planes. Also all out economy is 440 seats, which can be useful for Hajj/Umra season.
As you can see above picture, it's cheaper to operate a350-1000. The reason 777-9x is cheaper per seat is only if all seats are occupied, which is not possible for an airline like PIA since it's direct airline, rather than middle east which bring all people same airport, put everyone in big a380/9x and fly to london/jfk.
CS-100 at 50% discount can cost $30m. It can be good for Peshawar to Karachi or Lahore to Karachi or Peshawar to Lahore or Peshawar to Quetta. Seats are 120 for 2 class. It's also cheaper aircraft than a320 and more efficient than neo. Only downside is pilot and maintainance training.
