Plenty of people, people who are 50% one thing and 50% another for example, are half of each. The expression "I'm half Italian" or "I am half black" is not uncommon at all and it isn't meant as a racist or bad thing.
Being referred to as "Half," may be considered an acceptable colloquialism by some mixed folks who "reclaimed" it or were raised to not be offended by it, but it is considered very rude, triggering, and racist by many others.
But using the term "Half" is just a social colloquialism that is usually technically inaccurate. "Half" has a definition. Specifically, 1/2 or 50%. You may indeed receive 50% of your genes from each of your parents, but they themselves may not have been 100% each, and the percentages of DNA you received from ancestors at the grandparent level and further back are not necessarily neatly divided in two with each generation.
I would assert that it does not translate to 50% of "race," and therefore it should not count as "half" of species in the magical world of D&D (at least not in a majority for it to be the social norm.) In fact I highly doubt the many folks who have mixed heritage actually fit that 50% definition, mainly because prior generations have a variety of the their own mixtures.
That is why I support the terminology "Mixed" over "Half," and would be mostly OK with Wizard's first suggestion that it is mostly cosmetic (we humans are pretty hung up on cosmetics, though).
However I would LIKE to see "[species]" having 2 or 3 bullet point abilities that can be swapped in or out for variety, and having a "Mixed" Heritage or Lineage rule that explains how to do it.
In fact here is a suggestion:
- Every "[Species]" gets 2 major traits and 2 minor traits. (Or 3 and 3 if the design space warrants it, but every species would be designed in the exact same format.)
- This is easily represented by bullet points. I am also ok with some abilities being better/more useful than others (like a gnome's Gnomish Cunning vs. Darkvision) and being the most commonly chosen options by players. Those choices can reinforce positive stereotypical choices that create a baseline for people's expectations if that is what they want. But it does not negate variation.
- If you choose a Mixed Heritage, select two "[Species]" that represent your most prominent heritages, and you may swap out 2 traits between those two "[Species]." You may only swap major traits for major traits, or minor traits for minor traits. For the purposes of creature subtype, the character is considered both, for good or ill.
- (DDB could list the "[Species]" on the character sheet as "Mixed (Elf and Human)" or "Mixed (Dragonborn and Tabaxi)"
- The player can mix the cosmetics of those heritages as they wish, and in-game, can call their Heritage or Lineage whatever they want.
- For instance, in Eberron, those with mixed elf and human heritage often call themselves "Khoravar", and in Krynn a ranger born of conflicting cultures may struggle in either and call himself "Half-Elven" to reclaim it.