I don't think a second team in Melbourne should be a priority just now, but we've seen how well derbies are received and how much a bit of competition heats things up. Certainly, a second Brisbane would make more sense than a second Melbourne (bring back the Crushers for what it's worth).
That said, I read an article the other day where Eddie Jones suggested that in order to strengthen Australian Rugby Union, the ARU should scrap the Force and Rebels, just focussing on the Brumbies, Waratahs and Reds. I don't think that will happen, but.... if it ever does, the NRL should jump in there all guns blazing with their wallet out to bring the Rebels and Force over and change them into NRL teams. Ready made fans, similar sport etc. I imagine there would be a good rivalry between the Rebels and Storm (although some will be fans of both, that will take care of itself over time). It would be a huge kick in the guts to union and give us a head start with these new teams. My only concern is that those teams are centrally run as far as I am aware, so assume that the ARU 'owns' everything to do with them, in terms of licencing, branding etc.