This method really helped me with ideas for mine with was psychology based with a bit of sociology. Go to the module breakdown for a course you want to take at Uni and look at what they'll be teaching. It'll be relevant to your degree, interesting since you might be studying it next year and it'll put you ahead of the game when you get there (I can say from experience mine helped me on my degree).
Should prison be for rehabilitation or punishment? Is prison too nice? - Prisons and Punishment Core Module - Effectiveness of different forms of punishment. Critiques of incarceration. Alternatives to custodial penalties and their justification. The history and development of community supervision, penalties and punishment. De-institutionalisation and the abolitionist argument. Mental Illness in Prisons: An Investigation into pre-existing and created disorders in the prison system - Mentally Disordered Offenders What is the best way to rehabilitate victims of crime? - Victims and Offenders
Note: These titles may not be EPQ standard I just knocked up concepts from the page. :P Do not phrase them like this!
Extra tips: Use at least some statistics, a newspaper article, journal articles/experiments and your own experiment or survey looks brilliant. Message me if you like for extra advice. My EPQ title was "Assess the psychological causes of prejudice and discrimination in individuals and UK society"