Brainstorming apps usually = “wouldn’t it be cool if…” and then figuring out how to get the data you need to make your app work.
A much better way is to study available data sources and figure out a useful app based on what’s out there.
Check out: http://data.gov.uk/

My favs:
Don’t eat at
This New York- based app crosses Foursquare with food hygiene: connect it to your Foursquare account and it will alert you if you book at one of the 1,660 restaurants currently under health inspection. donteat.at

Tubetap
If a Tube trip takes 15 minutes longer than Transport for London estimates, you’re due a refund. Proving this is difficult, however, without the TubeTap app. You can time the journey and request a refund via the app. tubetap.co.uk

Source:
http://www.wired.co.uk/magazine/archive/2012/01/start/turning-data-into-apps