If they're going more fantastical, I'm thinking they'll probably go more futuristic than past era, for the feel of Metropolis. At least that seems to be how it's usually portryed.
They have options at any rate (and who knows, they may just plunk for the modern world as we know it without fantastical style places).
But if you go fantastical you don't actually have to choose between futuristic and 'period piece feel'. For instance, they go easily go retro-futuristic, ie, the feel of what people in the 50s and 60s thought the future would be like. Something that is both in the future and reasonably advanced yet also feels familiar from the past.