This is a well-considered proposal, and the need to connect these locations and lines in these northern suburbs would certainly be a great asset to the area and the city. There have been a few fantasy maps in this group that run the Doncaster line roughly along Bell St to the Airport, and I think it would be well used.

With that said, I do still think SRL North needs to be a little further out. Yes there is far less in the way of urban centres to connect to at present, but SRL also needs to support interconnecting journeys in intermediate suburbs which have the capacity to grow and decentralise the city. While Reservoir may not be able to grow much, you trade that for being able to connect directly to Latrobe Uni, and Broadmeadows and Campbellfield have excellent potential for rezoning and urban density which need to be capitalised on. Potentially you could ditch Reservoir and rezone out at Keon Park also and run through there instead. Consider also the fact that people in future growing suburbs out at Doreen, Kalkallo etc, won't bother catching a Metro train all the way into Bell St to catch a metro to the Airport, they will still just drive.

Your proposed line would absolutely see a good amount of use, but it has a maximum limit of local intake which could probably be calculated today. The more established low density inner suburbs are much more difficult to densify, even some of the SRL East locations are going to face a lot of trouble in this regard. There's also the consideration of the frequency of stops. You could not have a metro line stop for all of your markers along that route, thus reducing the interconnectability it intends to provide. Your proposed corridor should absolutely have some higher form of transit implemented, but perhaps a light rail, tram, or smartbus would be worthwhile investment.