This is a tough one because I'm a fan of a lot of things.

Probably something similar to Harvest Moon, only more to it than the normal "plant, water, harvest" type stuff. There would be special equipment besides the hoe, axe, scythe, etc, but you would have to do certain things to get them.

Sounds a lot lamer on paper

Another option is making an RPG, something similar to Morrowind and Oblivion because I love more involved games like that. It makes you think more.