Quoted By:
Everyone says Johto postgame, but while I think that would be cool, I dont think we'll get it simply because it would kill any reason to remake Gold and Silver down the line since it would essentially be the exact same thing in reverse, and they wouldnt wanna steal what made the Johto games so unique by having the other region.
I think expanding the Sevii Islands has tons more potential. From FRLG data, there are hints that they originally wanted to include up to 24 islands instead of just 9, I would love to see them bring those back, and that could potentially be the size of a whole new region if done right, especially compared to Kanto. New islands could be the perfect way to really fill out that dex with non native pokes.
They really helped diversify Kanto in terms of locations, we got a Volcano, an Ice Cave, a Canyon, Ancient Ruins, etc. I would love to see what else they could do especially if they start theming them. It could even have its own little Island Challenge with new gyms/trials like Orange Islands.