When it comes to mortality I made a sort of grid once, based off of my own ideas and the idea of 'immortals' in the books by Tamora Pierce.

Basically 5 levels of mortality

Lesser Immortal
Greater Immortal
True Immortal

Mortals = normal.
Lesser Immortals cannot die from old age.
Immortals cannot die from age or disease
Greater immortals cannot die from age, disease, or poison
True immortals are just that: immortal. Like the highest of gods.

However in my campaign lesser gods would be greater immortals with massive powers, so they could be overthrown, but it kinda takes followers and such. Ascension is such a cool goal. And the least of gods are weak enough they could still participate in the occasional adventure.