The caveat to everything I'll go on to say here is that there are myriad ways of approaching this sort of thing, and one method may not be any more "right" than the other.
For starters, try to find something you're genuinely interested in: this will make researching and planning a response much easier, and you will also come across much more authentically as someone who is genuinely interested and passionate. It doesn't have to align with the firm in some particular way - don't get me wrong, if it does then that's not remotely a problem, but don't try and force a link as this is unnecessary. You should, however, explicitly discuss the impacts on commercial law firms and their clients generally in the very least.
Next, think about how you would like to structure your response. Do you want to discuss one story in particular at length, or do you want to consider a market trend? Have a look at my interview prep article accessible via the link in my signature for a more substantive discussion on this.
Finally, on the point of "should it be something people are talking about"... well, yes and no. I don't remotely think your response should be based on something nobody has ever heard of: it should be of enough commercial significance that it isn't "new" to people interviewing you. Nor, however, do I recommend going down the route of something everyone is talking about as your answer risks becoming white noise. Try and find something significant but not at the absolute forefront of discussion.
Hope this helps as a starting point at least!