@Vaalingrade
@Mistwell
Note, in Mordenkainen, the traits for the Sea Elf are identical to the traits of the Triton, namely Swim Speed, Amphibious, Cold Resistance. The Sea Elf might really be an Elf/Triton multispecies. It might not be worth altering the traits of the Elf species because of the special case of the Sea Elf.
In any case, with flavor of "shapechanging adaptation" in mind ...
Looking at the design space of the Playtest Elf. It seems like a choice of "Elven Adaption" might work, each being worth a species feat. (Heh, turns out "adaption" is actually a word, as a variant of adaptation, albeit rare.) This Elven Adaption feature replaces "Darkvision 60 and Perception proficiency".
Elven Adaption
Elves inhabit environs across the multiverse, and many venture into the unknown to bond with new environs there. A child is born with the Adaption to the environment that one of the parents bonds with, but can bond with a different environment later in life. The transmutation is magical, but the form that results is nonmagical and enduring. Each time you level, you can swap your Adaption for a different Adaption instead. You can also use your level-4 feat to gain two Adaptions of your choice.
• Darkvision 120
• Darkvision 60, Perception proficiency
• Walk Speed +5, Climb Speed
• Swim Speed, Amphibious, Darkvision 60
• Resist 2 chosen damage types, immune when it is of a natural environment: Acid, Cold, Fire (=2), Forc, Ltng+Thun, Necr, Pois (=2), Radi
• Telepathy 60, Resist Psychic
Note. The above needs refinement but the approach seems doable.