During Superstorm Sandy US Airways and Jetblue sent a bunch of their aircraft to PIT. I’m guessing that will be a popular spot again.
Delta and their regional carriers have enough space at DTW, CVG, MSP, and ATL to put a fair amount of aircraft.
United will probably send some to IAH and DEN. They don’t have a lot of space to spare at ORD. They could also send some to CLE.
Florida may also be a popular spot - TPA has a number of hardstands and parking areas.
AA Ferry to PIT: flightaware.com/live/flight/AAL9 … /KBWI/KPIT