The Metrolink's need right now isn't more extensions. We just need more trams. I don't think M5000s are in production anymore, but the fleet is getting old anyway (the T68s were around this old when they got replaced) so it might be time to replace these trams, maybe with the tram-train vehicles that support future extensions.
Also, the network is slow right now, probably the slowest it has ever been. Temporary speed restrictions are everywhere, and you can't take a tram ride more than 5 minutes without the trams jolting to 5-15 mph at least once. To put it in better perspective, it has become so bad that extra vehicles are needed to make up for the time wasted, especially on longer routes, like East Didsbury - Rochdale, that isn't as punctual as it should be because there are a few single track sections that are too long, and the route itself is long (used to take 74 mins on average, now takes 85 due to 8 TSRs).
It is now even slower because the TSRs cause the trams to fall out of sync with the schedule, and therefore the timing for the single track sections, which was already tight enough with a six-minute frequency in both directions, gets next to impossible to keep.
So, even better than getting more trams, fixing the current network which is subject to delays, points failures, road traffic collisions etc.