That is an interesting question I have not come across yet.
I agree with Jessica, but guess this may be subjective.
Unless there is something that you studied in contract or seen in commercial news where you thought “this is not right”, I would recommend picking something that you are genuinely interested in. Not only can commercial topics be very complex (of course subjective), but if it is not something you really care about, you may also miss the chance to show that you are passionate about something.
I would just brainstorm the areas you are interested in (or directly involved in) & think about what issues there are - can be anything from environmental, social justice, public policy, workplace, renting etc. - and think about what law could resolve them.
I may be wrong but I believe that law firms want well-rounded individuals who are passionate about something, and they don't want you to only think about commercial law 24/7