I think I might merge the fiends (with maybe a little slaad) into one race, the daemons. Daemons can be of any alignment, though most are chaotic neutral, chaotic evil, lawful evil, or true neutral. Daemonic princes are worshipped as gods by some mortals. Hey, that reminds me. Gods.

I don't think I'll have "gods" in the traditional sense. The closest thing to traditional D&D gods will be hero-gods, which are ascended mortals. Dragons, daemonic princes, giants, and beast spirits are all worshipped at least a little bit and thus can grant divine power to clerics and paladins.

Also, dragons are kind of like angels, in that many serve hero-gods or even daemonic princes and they likely had some part in the creation of the world.