I know some of the niches they are looking for "more" of, but so are most schools to have a diverse student body. But mainly they are looking for the same thing they do always: very high academic achieving students. (Great grades, great test scores) with evidence that they are self motivated, can handle the self directed curriculum Brown offers, hopefully have some personality, and some sense of giving to the greater world. And then they try to have those students come from a mix of places, races, backgrounds, and a mix of where they might be headed and interests.
(The "more of" list are things that only a tiny minority of applicants are, so don't worry if you don't fit those "niches",
and include: students who are first generation college in the family, students from the "plains" states, students from Central and South America, students who are valedictorian high caliber (insert sport with alumni interest) players who will still play (sport) once they get to college, UMs who want to go to Brown for Brown and not just because it is an "Ivy". Hmm, I think that is most of recent wish list I've overheard in the past few years.)