I'm sorry, perhaps I'm missing what you mean, but can't you do exactly that now?
Using either Route Planning or Create New Route, you can create a route to anywhere using any plane, whether or not you actually own it. Then it's a saved route like any other, just not being flown, and all you've got to do is move it over when/if the plane arrives. The system even does the slot validation that you describe when you get around to moving the route over (as it will only hold a slot on a non-flown route for 2 months, so you can be putting a route on a plane that has no confirmed slots). I use this feature all the time when fleet planning. It's also useful for slot hoarding, as you can steal slots you don't have the planes to fly yet. Your costs go up though, as unflown routes incur some labor and slot costs.
The only thing you can't do is plot it out on one of the scheduling diagrams.