This is a big idea, no small undertaking I know. I'd like to combine my GPS app with the functionality of an app like Gas Cubby, which tracks my MPG at every fill-up and alerts me when my car is due for maintenance.

Here's how I envision it:
-I tell Waze what car I drive: ex: 2003 Saturn Vue, 6 cyl, automatic
-Every time I stop to fill up, I tell Was
a) What my current odometer reads (waze should know how many miles I've driven in theory, but this is a checksum validation in case I forgot to open the app while driving a few miles)
b) How many gallons I pumped
c) What price I paid
d) Waze automatically detects what gas station I'm at
-This allows Waze to track my MPG and cost of driving
-This also incentivizes wazers to use the gas price feature more often

Waze can then go several directions:
-Keep a database of real world avg MPG ratings for all sorts vehicles, a valuable database to have for people purchasing new and used cars.
-Waze gets more accurate gas price database because people use the feature at every fill-up
-Alert me of the best price of gas on my commute when it knows I'm low on gas (IE: it can tell I fill up on average every 300 mi, it starts suggesting places when I've gone 250 mi.)
-I can request routes based on new factors: quickest, shortest, or most economical route (since it can learn my MPG in city vs highway driving... it's tracking all of it after all). In many case I find, the route that's going to take me an extra 3-5 minutes to drive is also the route that will save me money on gas, which is worth it to me; but not always. It's hard to tell and waze could compute this for me.
-It can give an estimated cost of trip as well as ETA (EX: 20 mi to work, 26 minute ETA, $3.42 estimate cost of commute given current local gas prices and your driving habits and mpg)
-It can also alert me of other maintenance items: oil changes, timing belts, fuel filters, even to check tire pressure every couple thousand miles.

All of this would incentivize Wazers to leave the app open at all times when driving, whether they need directions or not, which in turn gives waze more accurate map and traffic date. This also gives a new aspect to the 'game' that is waze: I get to track and beat my own 'score' (mpg), and even 'beat' the avg MPG of other Wazers with my vehicle type.

I've been looking for a GPS that does all this for a long time, and there isn't a good one. Waze has the best chance of becoming that. I know it's a big request, but I think it would be amazing.