The temporary termini at St Werburgh's Road, Droylsden and Rochdale Railway Station corresponded to the limits of Phase 3a of the Metrolink expansion programme. Phase 3a, which also included the present line past Oldham, was approved before Phase 3b, so it's not too surprising that the lines initially opened to the points they did.
Phase 3b includes the extensions to Ashton, East Didsbury, Rochdale town centre and the airport, and the line through Oldham town centre.
The work on the airport line is not proceeding in a particularly north-to-south fashion. The last two stops to get platforms were the two northernmost, Barlow Moor Road and Sale Water Park.
However there are trailing crossovers south of Sale Water Park, Roundthorn and Wythenshawe Town Centre, so if they do decide to open it in stages, those are places which would be operationally convenient as temporary termini.