Firstly, I'd like to thank you for your contribution to the project. I mean your long post with game titles, found here: (john798 is my second account)
Of course, you're perfectly right about sandbox game ideas, which all seem the same. However, a description is still just a piece of text. I like the idea that new, unique things come to your mind, when you're doing, so I'm just doing. Did you know that at first Minecraft was called Cave Game and was an exact copy of Infiminer? Everything unique to this game came later and I think that's what gonna happen with Surviverse too.
About the idea of a castaway: When I reach the stage of creating Survival Mode, I'd like the plot to be similar to this:
Somewhere in the universe a high-tech civilization lives. You are a scientist and explorer on a huge spaceship, which gets crashed somewhere beyond the known cosmos. You land on the unknown mysterious planet. First, you need to survive. Then, because of your curiosity (you are a scientist), you start exploring the world. Soon, you discover minerals needed to build your known machines, but you also discover some sort of weird energy, which you think can be used and be even more powerful than any technology you know... As you acquire more knowledge about it however, weird things start to happen. You are in danger...
So yeah, you see that this is gonna be one of the main ideas.
But before further posting, please at least visit my website to get a full picture of things or just play this game to see what it is like.