All we need is an option saying "Take the traits from your character's parent species and pick X traits from among them according to these guidelines...", then set some rules to try and curb the most blatant abuses and a note making it clear the final trait list is subject to DM approval.
That's all it would take - barely more than a modest revision to the existing "Pick a Parent" sidebar and basically just a formalization of the system Critical Role uses in Tal'dorei Reborn.
It's not hard, we're just shackled by cries of "Suffer not the min-maxer!!!"
The problem is that the traits of each species aren't remotely balanced enough to do that. Every race has a primary "marquee" feature (flight, breath weapon, spells, shapechanging, stonecunning, lucky) and a bunch of features that are ribbon or weaker (free skills, darkvision, fey ancestry, energy resistance). The amount of the latter depends on the strength of the former.
Now, let's take a PHB option: the dragonborn/tieflng. According to the playtest, they each gain the following:
Dragonborn: darkvision, breath weapon, energy resistance, limited flight at 5th
Tieflng; darkvision, thaumaturgy, fiendish legacy (bonus spells plus energy resistance).
That's what you have to work with. Build a hybrid that isn't more powerful than its parents.
Hmm. Hard to do, isn't it? Darkvision is shared by both species, so that's a given. You can't give both spells AND breath weapons (the marquee feature of both races) and they both get energy resistance of some type. So the dragonborn tieflng is a dragonborn that trades limited flight for thaumaturgy? That's a bad trade. How about a tieflng that gets dragon flight at 5th for the cost of a vanity cantrip? Well, that's a strict upgrade to the tieflings kit, every tieflng will want a dragonborn baby momma. That said, you're still mostly at tieflng until 5th level, not very dragon/fiend, don't you think? I guess you could rip the bandage off and let them have breath weapon and free spells, making them superior to both dragonborn and tieflng, ensuring everyone who wants to play either race picks the hybrid option and full blood versions are relegated to NPCs.
And I picked two of the weaker options in the game. Good luck balancing changeling, aarakroca, aasimar, yuan ti, fairy, eladrin and other one trick pony races. The only way this works is dedicated stat blocks for each hybrid option to make sure they don't end up OP or redundant.
Or what WotC has already proposed.