I think that if they do offer "create-a-world", it'll be around the end of TS4's development since it is true that the worlds are major selling features for the new packs.
I'm someone who fills up all of the worlds and who plays with all of them in an integrated save. I'm sure that if something happens and I switch away from this save, I'll start again with a simplified game or a different theme. Anyway, the way that I play now, I use all of the worlds.
I can always use some more space. I'd definitely play with a new blank world if it were released. I'm also interested to see new themes such as the Mediterranean, tropical beaches and the Alps/in the mountains.With the release of JA, I'd also be interested in an African safari or another adventure in SE Asia or a TS4 version of Egypt. Obviously, I'd expect to pay for those.
I'd be happy to see them release another free extension of basic space though to extend a theme that we already have such as a blank extension of Selvadorada's town (minus all the jungle action in JA - just as a way to build on the GP).
