Come up with a list of Heritage feats that represent blended ancestry (or not). And make them Background Feats that can only be taken at character creation. I'll take a stab at the 2 most iconic, and one more (Abilities derived from Unearthed Arcana: Character Origins):
Elf-Blooded (Background)
Prequisite: Ancestry must not be "Elf"
Most characters of mixed heritage strongly favor one parent or another. Whether inherited from a parent or ancestor, or derived due to some magical condition, you share some of the defining traits of Elves, and gain the following benefits:
- Darkvision. You have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
- Fey Ancestry. You have advantage on saving throws you make to avoid or end the charmed condition on yourself.
Orc-Blooded (Background)
Prequisite: Ancestry must not be "Orc"
Most characters of mixed heritage strongly favor one parent or another. Whether inherited from a parent or ancestor, or derived due to some magical condition, you share some of the defining traits of Orcs, and gain the following benefits:
- Darkvision. You have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
- Relentless Endurance. When you are reduced to 0 Hit Points but not killed outright, you can drop to 1 Hit Point instead. Once you use this trait, you can’t do so again until you finish a Long Rest.*
Dwarf-Blooded (Background)
Prequisite: Ancestry must not be "Dwarf"
Most characters of mixed heritage strongly favor one parent or another. Whether inherited from a parent or ancestor, or derived due to some magical condition, you share some of the defining traits of Dwarves, and gain the following benefits:
- Darkvision. You have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of grey.
- Dwarven Resilience. You have Resistance to Poison Damage. You also have Advantage on saving throws you make to avoid or end the Poisoned Condition on yourself.