I'm sure we'll probably add some more domains in the future, but it's just not feasible to recreate the entire world. In part because of the game engine's limitations, but also in part because there would just be a lot of redundancy.
The Ravenloft setting was basically designed around "let's make a D&D version of a classic movie monster and build a country around it!" In a traditional tabletop "pnp" campaign, it's not likely that you'd visit every single one of these countries, either. I know that in my old campaign, the only domains I sent the party to were Barovia, Darkon, Souragne, Har'Akir, Sri Raji, Valachan, Kartakass, Gundarak, Forlorn, and the Nocturnal Sea/Nebligtode, and that was over a period of 2-3 years. Other peoples' campaigns might not go to any of the areas I listed, or might spend the whole time in a single domain. That's how things work in PnP: there's a whole broad world but most DMs rarely explore the whole thing.
But when you adapt the world to a NWN persistent world, each place you add has to have a reason to be in the game beyond "this is where our version of classic monster X lives" or "that's what's in PnP" or "it would just be cool to have." We have to balance things appropriately, we have to make sure we can reasonably recreate the content of the domain faithfully, and we have to make sure there's an interesting reason to go there (such as new crafting ingredients -- which then means we have to make changes/additions to the crafting system), and we have to make sure we're not spreading the playerbase too thin to the point that no one sees each other. Given that, there's no way to add the whole entire world map in a practical way (and we'd have performance issues if we tried to do that anyway).