I don't think the Cetra thing could work since Black people still exist in FFVII. Although they already drew an obvious analogy between American corporate imperialism (Shinra) vs Eastern or Indigenous cultures (Wutai), Corel itself, with its high concentration of Black people and desert terrain, could be a parallel to Africa and how it has been ruthlessly exploited by Europe/America. Also, since Barret is black, people could mistakenly draw the conclusion that he is a Cetra. Thirdly, in Japan, mystical types are usually depicted as Indian-looking, so that may be a better option. Then again, that's kind of a stereotype. As a last point, American race relations are very different from the rest of the world, especially as it pertains to Black people. Every country has its own set of complex relations, many of them more heavily based on culture than race (America is
obsessed with skin color by comparison), so I don't think we should dig too deep into race beyond aesthetic representation. Culture is more pertinent here. Shinra imposing on local cultures is obvious.
A black Marlene would be interesting. In the original, we knew that there had to be a story behind Marlene since she didn't look like Barret, provoking curiosity. Making her black would eliminate that curiousity, but it would increase the surprise factor of Barret's backstory for new players.
Edit: Damn, I got ninja'd by so many people.