I voted Greece (although an extended Roman theme would be equally nice) because I would simply love a Grecian theatre and a temple: I'm thinking
tourist attractions for my clickies going on holiday.
But if I could choose a theme myself, I'd go for a fair theme: Food stalls, a ferris wheel, roller coaster, arcade games, caravans, a fortune teller, animal rides...
I know I wouldn't be able to resist buying all of that
Or it could be a Victorian fair, with older style attractions