Outwood Lane bridge is a problem, in an ideal world it would span the 50 metres across the station to allow for rail expansion. The other abutment between platforms 3&4 is awkwardly placed as well. You could potentially slot another platform there - potentially 2 with some creative realignment.
Huh? Do you mean the abutment between P1 and P2? The P3 and P4 tracks are immediately adjacent to each other.
We are where we are, due to decisions that made sense at the time. Originally Manchester Airport station had only two platforms, now P2 and P3, either side of the large central island. When the third platform (now P1) was constructed in 2008, an additional pre-cast bridge span, complete with abutments, was constructed to the side of Outwood Lane and then slid into place adjoining the original southern abutment, to minimise the road closure time. Then, when the Metrolink line and P4 were later constructed on the northern side of the station, the original bridge span and northern abutment were demolished and replaced by a longer span, supported by a new abutment between P4 and the Metrolink tracks, and by the pillars on the P2/3 island. The original southern abutment and the P1 span were left unchanged. Again this construction method minimised the road closure time.
During both construction projects the existing platform alignments were kept unchanged, to minimise disruption to rail services.
If the station and bridge were completely demolished and remodelled, clearly more platforms could be fitted into the existing footprint and the timetable conflicts eliminated. But the disruption, and cost, would be horrendous.