If you're going to do something like this, might I suggest you start with two lists, one is the dominant race list, and it's the base race's features, where you select a feature (or two) to replace, and then dilute some others, the other list would be the subordinate list which would have the feature of the race that you can add to the dominant race. I'm not implying that some races should be automatically dominant or subordinate, I mean each list would contain each race that is interbreedable, and then one list is where the bulk of the race features come from and the other is the bonus feature.
So, say you want to breed Orc and Human, already done; they get the darkvision, savage attacks, and relentless endurance of orcs (the playtest orcs I saw had relentless endurance), and the human aspect is menacing which grants them proficiency in intimidate. Say you want a Half-Orc Half-Dragonborn, half-orc dominant, they drop menacing and pick up Draconic Ancestry, except they only get the breath weapon (would you rather grant them all of the other half-orc stuff and resistance, or let them get a small AOE that costs an action and recharges on rest). Let's say you want a Half-elf Half-Dwarf (for the lols), half-elf dominant, grab the half-elf, and trade out the bonus proficiencies (since that's what SCAG does) and throw in Dwarven Resilience (can't see this character getting trained by dwarves so combat training and tool proficiency seem right out, other features are subrace features, which you may want to require rather than general dwarf features, dunno). Let's say you want the half-elf half-dwarf, but you want it to be dwarf dominant, take all the dwarf stuff and drop combat training and tool proficiency for fey ancestry (immune to sleep, adv vs charm). The former moves at 30 base (since it's elf dominant) and the latter moves at 25 (since it's dwarf dominant).
It would be quite the undertaking to go through all of the races, two times, and determine what would be most appropriate to switch out, while still keeping it balanced, but man I bet it would see a LOT of play (and probably sell pretty well for a buck or two a pop on the DMs Guild).
The real question is, do you want to go a generation further and have hybrid hybrids, with super diluted racial abilities, but maybe an extra stat boost (keep things like darkvision but drop most everything else, get an extra ability point depending on race, so a half-orc/half-dragonborn hybrid fell in love with a half-elf/half-dwarf hybrid, creating a Orcgoneldorf (or some kind of crazy combination) that gets +2 Str +1 Dex +1 Con, darkvision, fey ancestry, savage attacks, stonecunning, and a breath weapon (three of those races can grant +2 strength, only one offers +2 con, and only one offers +2 dex, other stats are at +1 bonus so can be disregarded). That would be an insane amount of work, not even considering the effort it would take to balance it.
