It's.... always been one island/village/town per system. This is nothing new.
As per the creator, Katsuya Eguchi;
"Another thing is that I'd always get home really late. And my family plays games, and would sometimes be playing when I got home. And I thought to myself – they're playing games, and I'm playing games, but we're not really doing it together. It'd be nice to have a play experience where even though we're not playing at the same time, we're still sharing things together. So this was something that the kids could play after school, and I could play when I got home at night, and I could kind of be part of what they were doing while I wasn't around. And at the same time they get to see things I've been doing. It was kind of a desire to create a space where my family and I could interact more, even if we weren't playing together."