To me a human with fey traits from magic and a magical pairing of a human and elf are represented differently.
Why different mechanics? Gaining the traits from an other species is the same end result. It doesnt matter how it happened. (Gaining DNA from sexual reproduction or from gene splicing, is a mix of more than one parentage.) Did the Dragon species come from a Dragon distant ancestor, or from a neighboring Dragon? Both are a form of parentage.
Feats are a solid design space to explore various posterity concepts.
That said.
The species design space comprises about one and a half level 4 feats, equivalent to three background feats. Here, I will refer to these as three "trait feats". Every Player Handbook species has three "trait feats". Plus the free background feat, every player character starts of with about four half-feats of design space in total. This is an enormous amount of design space to build a character concept.
If the traits of every species bundle into trait feats, most species comprise three bundles. For example for the Elf, call the trait feat that has the Darkvision and Perception bundle the "Nocturnal Spirit" trait. Then it is easy to swap out an unwanted trait feat for a different trait feat from elsewhere.
A Dwarf-Elf concept might swap out one of the three Elf trait feats to gain a Dwarf trait feat instead. A Human-Elf might swap to gain an extra background feat to represent a Human parentage.
A player can even swap out an Elf trait feat to finetune the concept of an Elf. For example, I dislike the Elf having Darkvision and Perception. It makes sense for a nocturnal Wood culture, but less sense for a Sun culture. So I might swap out this "Nocturnal Spirit" trait feat, for a different elven theme feat, maybe Lucky.
Some traits are powerful, and are worth about two trait feats, such as Fey Teleportation per Short Rest. Perhaps flight is worth all three trait feats.
Because of balance, the players and-or the DM need system mastery when mixing trait feats − in order to notice and avoid a broken combo, if any. But it would still help when the core Players Handbook lists the species traits in discrete trait feat bundles. At least then the DM can easily create or permit a variant arrangement of trait feats, for the concept of a specific player character.
As a player, I want freeform customization to build any character concept. As a DM (and as a player), I want all the player characters at the table to balance with each other. I want to doublecheck any proposed character concept that has mixed trait feats, then give it approval if it seems to balance enough. This need for DM approval for a "variant" mix of trait feats, seems ok to include in the core Players Handbook. Alternatively, mention the possibility of mixing trait feats in the DMs Guide, but make sure the Players Handbook lists the traits in trait feat units.
Organizing the species description into trait feats, makes it easy for the DM to worldbuild, by customizing a new species that emphasizes the themes and tropes of the setting.
