I feel like I've read a thread in these forums with nearly the same topic, but I think enough time has passed for some fresh ideas to pop up. I think the extreme popularity of Minecraft really ignited a lot of imaginations, and with games like Day Z finally rolling out, I don't think it'd be inaccurate to predict a sort of golden age for sandbox survival games.
I bring this up partly because I'm nearing the completion of my first project and have begun thinking about my next one: a virtual ecosystem set in prehistoric Africa, in which warring tribes compete for resources and land. A great deal of my time will be spent on simulating real flora and fauna of the savannah biome. I love animals, and I love the idea of evolution by natural selection. To be able to realistically model an animal's behavior as it interacts with other animals and its environment would fill me with incredible satisfaction, and I look forward to fully fleshing out this idea in the coming years.
The player would assume the role of a single tribesman (or woman) with the ability to switch control to any member of the same tribe as long as they are within close proximity. The goal would be to explore, survive, and expand. Likely in first person.
What simulation setting would most excite you? Dinosaurs? Alien worlds? Railroads?