For a decent game you have to have massive amounts of customization; races, classes, appearance (male/female), many items which change the appearance.
I think Heaven, Hell and Earth work, I'm sure people aren't going to care that much. If anything, super religious folks, or lack there of may like the aspect of being able to defend their belief.
The whole flying thing is over-rated in my opinion, unless the world is seamless, and you can fly anywhere you want, it is dumb.
You should also have mounts that are ridable on the ground, and mounted combat.
A heavy crafting system which is actually entertaining, and try and make your game actually have an economy.
But most of all, make it very customizable, as in appearance, with many different skin colors, hairs, hair colors, clothes, masks, hats, shoes, armor, the list goes on.
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