For me personally I would have to say that #1 is a Western. It seems like it needs to be done. Red Dead Redemption really makes the fact that this hasn't be done sad.
- Western: Red Dead Redemption style.
- Post Apocalypse Fantasy: A world setting where a technologically advanced civ based on magic goes backwards due to cataclysmic event. Mix of low magic tech and traditional fantasy.
- Modern Fantasy
- Grimy Dark Ages with no magic
I would say those are my top 5 right now.
EDIT: Forgot an exploration game one huge server world with a lot of focus on trade, travel, resources and exploration. A sandbox like discovering the "New World."