If not Merseyrail/Metrolink split with a meet at Warrington Central (my vote for various reasons of frequency, uniformity in stopping, metro-ization/de-carbon and release of paths at both ends for other services) - - - - then I think this line works best for local services.
The issue is that no great frequency can ever really be realized (and it should be 4-6tph in reality, it would be, say, in Japan - two patterns nicely flighted) - because the Manchester end is a mess. Either another terminal point could be found - for example both platforms at Oxford Road (5 and 1 - but issues with latter) - or maybe a Cornbrook solution could be made, again to add frequencies and feed the Met.
Sadly due to Castlefield, I don't see this ever being as useful as it would be on either metro network at their respective 4tph and 5tph patterns. The long distance services should be re-routed.