Well, Yoshi said they had to chose between flying and swimming for the expansion, but that means the idea has been delayed not rejected.

Yoshi said he wanted to implement a lot of features, we don't have confirmations and most of them have never been mentioned again, but he wanted to add swimming, flying mounts and take out the invisible walls, so who knows... would be great to have a "roadmap" with the content they have planned for the next 4 years.