Which villainous race(s) are next on the chopping block?

[Naanomi]

She was the first being Asmodeus convinced to join him, before the creation of the Nine Hells. A powerful angel who guarded the gate as he recovered from the process of forming the Nine Hells.

Note that in official lore, Asmodeus *absolutely* did not create the Nine Hells... it (probably, depending on which backstory of Asmodeus we accept as true) preceded him by uncountable eons. It came into existence 'naturally' (or, according to the laws of the nascent Outer Planes as they developed); with a little help from the actions of the Baernaloth, perhaps Ahriman, and maybe the presence of Piscaethces. A whole race of fiends occupied the place and then... something... and were (mostly) gone before he took over.

It frames Asmodeus (and other Planar Rulers) in a much different light and role than default expectations if he can create new planes of existence (just as it did in 4e when he somehow threw an entire infinite plane into another one... justifying it by saying he became a God, who *also* don't have the power to do anything like that... something 5e swept under the rug and quietly retconned away). He is powerful, supremely important in the cosmos; but not a world-builder in that way.