I'd be interested to see what Waze could do if they partnered up with people that manufacture devices that plug into an vehicle's OBDII port (see the port under the steering wheel of vehicles produced after 1996). I've used one from GoPoint (http://gopointtech.com) but their available app(s) don't really overwhelm in terms of design and functionality and don't run in the background.
There's some amazing data that goes unused every time you use your vehicle. I'd merely love to see an opt-in option that could tie into to a devices such as this that could provide Waze with even more precise data than just determining your speed and position and direction based on your GPS.
Running Low on Gas - Given the recent push for gas (petrol) stations into the Waze system, being opted in while using an OBDII scanning device would make that data even more useful. Imagine setting a desired alert level for your vehicle to be notified that you're running low on fuel and be prompted to see nearby fuel stations.
Vehicle Troubles - If your onboard vehicle's computer throws a check engine light you'll know exactly what code is triggering the alert on the dashboard and can see an explanation of what's causing this to be triggered (or possibly have the error description spoken via the device's text-to-speech capabilities). In addition you could be prompted to search for nearby auto repair locations or tow services to get you there should it be a critical error code.
Track Fuel Economy - Waze could use your user account name and save data to the cloud in terms of your vehicle(s) fuel economy so you know better how your vehicle is operating. Have a plotted graph that shows how your efficiency has been, maybe Waze could tie in the efficiency data into routing considerations if relevant enough.
Gauges HUD - Your vehicle only shows a few primary gauges, what if Waze gave the option to display a gauge of your choosing (presently only speed is an option).
Overall Types of Data Collected
There's more than enough more relevant stuff that the Waze team is presently working on, but merely wanted to offer this up as an idea for some point in the future to add data to routing and the driving experience that I don't believe anyone else is offering.