Unless the LIV situation is resolved, GMac won't have a sniff (same as the other early defectors like Stenson, Poulter, Westy, Casey, Garcia, Kaymer etc.)
Harrington will be 56 and too old for the current 'profile' (plus it's highly unlikely a losing captain gets asked back)
Franceso Mollinari could be an outside option but if Donald wants it again (and why would he not, it's a great job) I reckon he'll go for the 'three - peat'