Now that's an open question. Hundreds of interesting topics are CJ related. Global security comes to mind among others.
adjudication,bond, bail bondsmen, court proceedings, defense counsel, electric chair, felony,guardianship's, high security prisons, interrogation techniques, double jeopardy, mandatory sentences, no lo contendre, and many more.
I find interrogation techniques interesting. The skilled interviewer can interpret body language, facial expressions, intonation and voice patterns to help him arrive at the truth. Interviewing suspects is just as much a weeding out process as it is an effort to find the bad guy. An exact science this is not, but, more and more detectives and the like are learning and then employing these techniques to assist them in the search for the truth. Most criminal suspects and defendants want to tell their story and will if given enough time.
Remember too, that criminal defendant's have rights that must be observed by the interviewer and a failure to do so will jeopardize the case.
Assuming the parameters of your topic choice are actually this broad, I would choose to analyze the popular viewpoint that the justice system is racist or classist in favor of whites and the wealthy.
The idea is that there are a disproportionately high numbers of ethnic minorities in prison. Is this true? If it is, why is that so? Is it because minorities on average tend to have higher poverty rates, and therefore commit more economic crimes? Is it that minorities are more likely to have limited finances and are unable to hire competent or dedicated lawyers, relying instead on overworked, overcommitted public defenders? A mixture of both?
There is also tons of statistics and research material available on this topic from the Bureau of Prisons or the US Census.