Hi Wfpk4life,
There are no plans to expand the islands that I am aware of. If players wish to have additional resources in their game, they can replace an island with a custom island they built in the cartographer, but the number of islands will not change. (The island the player is on when in single player mode is marked in red, as well as the boss islands, these can not be replaced)
While I do not know if the team will plan to upgrade graphics or visual elements in the future, digging, caves and additional wildlife would be a feature set and likely will not be added to the game. The team have confirmed the PC version is now feature complete and they will be working on issues reported by players for now. That said, in a recent Q&A that Sam filled out he did hint that he’s been thinking about the next game… [PC] Message from the Team, 2022-09-19 whether it’s a sequel or not, or what features it may that Stranded Deep does not remains to be seen…