Hypothetically, near future transhumanism:
A consciousness could exist in a virtual reality − a thought construct without flesh and blood. Then print out a body, and transfer the consciousness from the virtual reality to the new body.
The material body can have a human DNA, and be a human-species construct, and sexually reproduce offspring with natural humans.
But the virtual reality consciousness would be a nonhuman species.
In other words, a conscious mind can immigrate from one species to become a member of a different species.
A more competent scientific understanding of how consciousness itself works might come with its own implications about what a lifeform is.
This hypothetical transhumanism is actually closer to how Norse animism works.
The natural world is conscious. Just like a human body is a feature of nature that has its own consciousness, every phenomenon of nature is its own body with a consciousness. A mountain is conscious, a field, a lake, the sky, the sun, etcetera.
It is possible for a member of one kind of nature being to emigrate to become an other kind of nature being. For example, at death, the consciousness of a human nature being immigrates to become a member of the corpse nature beings. But it is also possible to become a member of the æsir nature beings in the sky as one of the einherjar among the clouds. There are examples of humans becoming vanir, jǫtnar becoming humans, and so on. Immigration from any species of nature being to become a member of any other species of nature being is possible. Essentially, this is a transfer of consciousness.
This Norse animistic worldview influences my perception of D&D fantasy settings. For example, the original Elf is a Celestial thought construct, whose consciousness translated into Fey force, and even Material body. The High Elf is an example of a Material Elf culture. The High Elf body is humanlike and can even sexually reproduce with humans. In other words, the body that Elf materializes actually is a human body with human D&D. The High Elf actually transfers the Elf consciousness into the human genepool. At the same time, the Elf maintains an affinity with the other modes of consciousness, including Fey and Celestial. The Elf consciousness has the potential to revert back to the nonhuman species. Perhaps the Elf Trance feature is a method to maintain the immaterial aspects of Elf consciousness.
Likewise, it is possible for a human consciousness to immigrate to an immaterial species, to become a Fey spirit-force consciousness among the Eladrin Elf, or a thought construct among the Celestial Elf. Perhaps an aspect of this Human-to-Eladrin consciousness maintains an affinity with the material species of humanity.
I like the One D&D approach. A character can be an entanglement of any combination of magical creatures. But pick one of these species for the mechanical stats for the character.