My top two choices are Japan and Spain, either as neighborhoods or as vacation spots. I know Japan's a popular choice, but Spain just has this beautiful culture and style that I'd love to see. Jungle Adventure is kiiinda similar to Spain, but there are enough differences between Spain and Latin America.
Portugal would be a great vacation spot, too. You have hot springs, beaches, castles, farmland, festivals, a busy night life, lots of festivals, and some choice scenic locations. I don't think they'd incorporate all of Portugal into the game, but there's a lot of options for content here.
I'd also like a nice, rural midwest world. Something down-to-earth and cozy, for simpler sims. I've been using Oasis Springs for that, but I'd like somewhere a little more green, too.