Your idea does show some weaknesses in current route planning software that you could address. But it doesn't seem strong enough idea to build a whole new application around. If you could layer it on top of Google Maps or another mapping product such that the full functionality of underlying product is still accessible and your super-route planning stuff elegantly overlays, then maybe you'd have something. But I'm not enough of a mapping web application guy to know if this is possible.
Whatever you do, keep in mind that whatever you base it on could add your functionality to their product at anytime and undercut you.
I guess I'm saying you've got a good mapping feature idea, but not a good new mapping application idea.
Take a good look at Streets and Trips which has layover capability. I never used the feature, but it may already be able to do what you are envisioning.