Lots of cities in Europe has they local carsharing providers. Usually they recommend waze as a navigation service. Would be cool to see support for those in waze: e.g. start/stop rental spots on the map, cars availability on these spots, probably navigation options to the nearest one and maybe later even a counter with rental time.
Rental spots may be integrated the same as it is done with Gas stations of a certain brand but with the live info about available cars on these spots.