After reading Trollses' Castaway thread, I see I'm not the first to have the idea. Don't worry, this will be much different. Without pulling out my notes, here are a few features I plan on having included.
-Day and Night cycles
-Dynamic Weather (effects things such as firestarting in the rain)
-Encumbrance System (I'm counting on you ForeverZerO!)
-Nurtrition Meters (Not just Hunger, but Protein, Fat, and Carbs)
-Skills: Shelter, Water, Fire, Woodcraft, Stonecraft, Bonecraft, Tracking, Stalking, Trapping, Fishing, Foraging, Agriculture, Weaving, Pottery, Tanning, First Aid, Cooking, etc.
-Abilities: After reaching a certain level in an appropriate skill, abilities will be learned (i.e. After reaching a certain level in the Fishing and Trapping skills, the Tip-up ability is learned.)
-Karma (still figuring out a good name for it) System: Certain actions will be either possitive or negative with the native spirits. The players' rapport with these spirits will effect the outcome of various events and actions.
-Other ideas I can't think of at the moment.
Either way, I plan on making this very original. Maybe not even technically an RPG.