One possibility of what could devas be are aliens, like the approach Marvel is taking with the Norse Gods, Thor being an alien race which explains his superpowers, long life etc, due to superior tech which is indistinguishable from magic (to us).
One disadvantage of thinking aliens must (only) be devas is that there could be some monks coming to a prediction that we will never find aliens as devas always likes to hide from humanity. If an alien race was found in the future, that would look bad on Buddhism.
So let’s say we have some normal kind of aliens, biological, carbon based, DNA, RNA, using water as medium of chemical exchange in their cells, having a brain etc, they need not be humanoid shaped (like Chewbacca from star wars), but they can be even octopus shaped. They have civilisation, tech, science much like ours, not vastly superior.
What would you classify these aliens as under the 31 realms of existence?
I put them under human realm under the following considerations:
Human realm need not refer to homo sapiens species. There were other homo species which when extinct, like homo neanderthalensis. So human realm need not be tied to specific species, and can be applied to other life forms in the universe.
Buddhist classification of the realms is in general humans experiences happiness and pain, whereas the lower realms have more pain than happiness (except for some pets), the higher realms has more happiness than pain. Given the aliens are like us experiencing happiness and pain, I think they are closest to human realms.
Since the Buddhist model of cosmology is cyclic, the human realm had been destroyed and reevolved countless times before, not every single cycle we might look humanoid in shape, depending on the evolutionary pressure, laws (constants of nature) of the new universe etc. We can apply the same consideration to the aliens.
There could be grounds for classifying aliens as asuras (yakkhas, nagas etc), if they have supernormal powers compared to humans and are war like in nature like the Klingons.
It’s an interesting question to think of because only humans are allowed to be ordained. What if there’s some aliens posing as humans?
Or to what extend if humans have non biological implants do we consider them cyborgs instead of humans. Clearly spectacles are no issue, how about surgically implanted hearing aid? Artificial heart? Artificial limbs? Brain neural connection got replaced by chips little by little?