The game designer wrote about this on his Twitter – it seems that this is his first mention of working on anything other than Death Stranding.
Crustaceans and marine animals in general are in the spirit of Death Stranding, so there is hardly a hint in the record about what the next game will be about.
The fact that the game designer is already thinking about the future is quite natural, since Kojima had said more than once that he wanted to make games until the end of his days, but he was not sure how much time he had left.
Death Stranding will be released on November 8. Perhaps if Hideo Kojima is already starting to prepare for the next project, this means that no additions to the game, at least the plot ones, are planned.
At TGS 2019, Death Stranding will show a total of 80 minutes of content (September 12 and 14), including gameplay.