I really don't think making your marginalised werewolf character black is a good shout - he's poor and shabby and cast out because of what he is, treated with mistrust everywhere and filled with self loathing and shame. If people think James et al bullying a wannabe death eater at a time when Voldemort is killing muggleborns makes them look racist because the death eater happens to be black, then the same logic applies to the whole wizarding world being racist to Lupin and the way he has internalised being a werewolf becomes about his internalised racism. If anything, it detracts from his actual problem, while making everyone else look like the National Front.
Book Snape is a cunt. Harry doesn't like him because he is a horrible bully towards him - and that's OK, whatever colour Snape is. By the time we get to Snape's Worst memory we will have had five years of watching Snape be a cunt to the kids and will know he used to be a Death Eater. I think James is actually fine to use Snape's own spell to dangle him upside down within the context of a war on which they are on different sides (and James is on the good side).
I dont think Paapa Essiedu was a cynical, diversity hire. By all accounts he is a great actor, and Snape is an amazing role for him to take on and the opportunity of his career. I would hate to see any other cynical diversity casting to try and offset Snape now being black.
Just let him be black.
If the best person to audition for Sirius / Remus/ Peter is black/ Indian/ Chinese, so be it - give them the role. But they shouldn't be specifically looking to tokenise one of them just so that viewers feel more comfortable with them being shitty towards Snape.
They weren't racist in the book. They won't say or do anything racist in the TV show. Snape was a blood supremacist and they hated him. They were kids who treated him badly. It's up to viewers not to handwring and pretend it's something that it's not because they cast a black man as Snape.