Some sources of free ideas:

http://ycombinator.com/rfs.html

http://ycombinator.com/ideas.html

http://androidforums.com/application-requests/

http://www.judegomila.com/2012/01/challengeyourself-in-2012-...

I think you're still going to be guessing though. It's good that you recognise that as you should then move to validate that guess work somehow. Good luck!