Level Up is mix and match.
I suppose a point buy system where you can create a custom lineage could work. It wouldn't be a "half XY" system per se, but you could take a CL and modify it to give it certain traits and flavor that up as a mixed species. You probably won't get aarakorca wings or dragonborn breathe, but you could probably get darkvision, fey ancestry or powerful build.
Where a general feat at level 4 is worth about 8 points, the background feat is worth about half this at 4 points.
Each skill, tool, and martial weapon, is nominally worth about 1 point each. But some skills are better than others, etcetera. Perception is worth about 2 points, and the "good" skills about 1½ points. The tools are still uncharted territory, but one can assume that some are better than others. Unfortunately counting each musical instrument as a separate tool is substandard − at best worth ½ point, like each language, pretty much worth 0 points, and is gated in separately.
Darkvision is worth about 2 points, equivalent to some of the best cantrips.
Traits can be organized into feats. And smaller ribbon traits can bundle together to form a feat.
Then each species lists its traits in units of feats.
One background feat for Elf is:
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Innate Magic. As an Elf you innately wield magic. Starting at level 3 and again at level 5, you gain the ability to cast a spell with this trait. You cant cast the spell again until you finish a Long Rest, however you can cast the spell using any appropriate spell slots you have. As part of your innate spellcasting, you can choose any mental ability for your casting ability, whether Intelligence, Wisdom, or Charisma, and you never need to use components for the spell. Different elven cultural lineages tend to rely on certain spells. See Elven Lineages table to determine the spells you gain.
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This Elf trait is also a background feat!
A Human character can use the extra feat to choose this feat to represent an elven parent or ancestor.
Meanwhile a character who descends from more than species, can swap and mix-and-match these trait feats to represent the ancestors and the individual character concept.