I have one idea that I will probably never have time to do (I have 3 childs and a fulltime job). It is an app to manage fuel consumption. The idea is that some people use a notebook to write each time they buy fuel (I did until I bought a Nissan Leaf). If the application is as handy as a notebook (maybe a simple picture of the gaz pump would be enough to fill in the data), some people will use it and you will start to collect the price and location of all gas stations. Once you have enough data, you can provide additional service to find the best gas station taking into account your position, current destination, remaining fuel, ... Gas stations may want to pay you some advertising, ... The application may grow further to take into account maintenance of the car. Maybe it is a silly idea, but if you build it, I would use it (for my second car).
After looking at the screendumps of current apps, it seems they still are awful. IMHO, a good UX is to have one click to take the picture of the pump, then you can check the fields. The mileage field is automatically filled using your average consumption, you just need to fix the last digits if you want, then press OK. It should work fine even offline and synchronize latter.
