im going to try to be more deliberate in the way i answer this because i feel like ive been vague/disinterested when its been asked before. the weaknesses that ive seen come out from other earth cs are that they are Our Earth, With Whacky Paint On It. which!!! is already what earth in homestuck was. this is a world where icp were the presidents and betty crocker is a fish alien. having Troll McDonalds and shit is funny and definitely on theme with what homestuck was - so depending on what kind of mood youre going for it really isnt even a "weakness" at all. for example, if earth c is Whacky Species-Segregated Earth, what does that say about how the author views the nature of humankind? are they commentating on how a bunch of socially isolated undereducated children were allowed to have a hand in founding society and therefore stunted its growth in the same way their own development was stunted? is it really all that shocking that the world was painted in black and white, or that capitalism became a thing again, or that cities look exactly the same way they do now? i do think theres value in earth c stories that explore the theme of "the more things change the more they stay the same." cycles and time loops are a big thing in homestuck after all. but if your roxy drives a sedan and lives in a suburb and youre not going full hog with why earth c has failed to outgrow the world it sprang up from, what are we even doing? did you just not care to put imagination into it?

i dont want to make it seem like, oh, just because i poured hours into pointless worldbuilding that mine is better, or that an earth c story set at Carapacian Kingdom Dennys is lesser... but i do think its kind of a nothingburger. im the type of person who is always taking shit too serious and thinking too hard about the worldbuilding in stories where we arent supposed to question it. two thousand years have passed since society was founded on my earth c - thats the same time between us and the roman empire, so if a similar timeframe has passed on yours, why is everything exactly the same? did every kingdom seek to replicate the perfect image of what their society was like before the game? were they hoping the gods would come back and live among them if the world looked more like their own home? that would be a good explanation! if your roxy NEEDS to drive a sedan, maybe auto manufacturers are bound by law to stick within the parameters of old earth. thats a stupid law, and its funny, and it still allows for a chance to worldbuild

maybe this hasnt answered your question as much since you specifically asked what part of my earth c id like explored. thats a hard question to answer, but if i had to pick one it would be religious worldbuilding. therapystuck has already made very promising references to this already, and i lapped it up like a dog. honestly as long as youre trying to make it visually unique and its clear you give at least a little bit of a fuck about it other than a backdrop for homestuck characters to kiss, then thats great. but if youre going to make it Earth But Make It C, then at least commit to why everything is so stagnant