What I'd really like:
1. You open the app and press a button. 2. A car arrives at your location within a few minutes. 3. The driver hands you a key and leaves. 4. When you're finished with the car, you open the app and press a button. 5. Within a few minutes, a driver arrives and you hand them the key.
This could have plenty of optimisations. For example: I probably want to go away from home and then back home. It might make most sense for me to have a pre-arranged schedule for receiving / returning the key. Perhaps I could have a regular booking on the app.
In the future, maybe it's 'keyless' and it uses self-driving to get to me. Maybe it drives itself while I'm in it, and just goes and parks itself somewhere out of the way / free when I'm not actively using it. When I get 'it' back, maybe it's a different car - that's fine.