In my campaign I have 3 categories, Elder Gods, Gods and Incarnates. Mortals are never going to ascend to the Elder God and God states, that's just the way it is. My Incarnates aren't quite gods, but are more powerful than standard outer plane beings. Orcus, Grazzt and Demogorgon would fit well in this catagory (though I haven't decided if I'll integrate them into my cosomology). One of my Incarnates is an ascended mortal, so I've implied that it is possible, though it is not something common and the mortal must be somewhat "special".