Ideally I'd like girls in the 'Manage Custom Girls' area to remain templates with zero affection & zero weight gained. But I do have something similar to your idea in the works for the next update.
I'm adding a new game plus where you keep your level, upgrades and money but move to a brand new town with a clean contact list. You'll be able to take the train back to any old towns to visit or move back. I'm also considering adding the ability for a girl to move in with you and then move towns with you if you so wish.