Removing racial ASI removed incentive to only play races where your bonuses align with your class. It made it ok to play a flavourful but mechanically inferior combo (like a shadar-kai shadow sorcerer). That is loads different than picking which two species are going to give you the best advantages and saying "those two had a baby".
There's not as big a difference as you're saying. Picking a race/ASI package just to minmax isn't much different than picking a race/race combo just to minmax. If anything, the latter is more work.
As to the second, you walk a fine line between powerful and flavorful. What to you speaks to an aarakroca: flight, talons, or gust or wind? How about a changeling: shape changing or social proficiencies? You could certainly say a changeling/aarakroca gets talons, gust of wind and two skill proficiencies, but does that say "part birdman, part shapeshifter" to you?
Let's go to Level Up for a moment.
In Level Up, to play a person of mixed heritage, you pick one heritage's traits and a second heritage's gift--each heritage has 2-4 gifts to choose from. So a hypothetical aarakocra/changeling PC would pick the aarakocra traits and one the changeling's gift. Now, I created a changeling-esuqe heritage, the Mimickers, for my
Handbook of Heritages (
not an affiliate link; I forgot how to do that) which had a single-form shapechanging as a heritage trait (you can switch between your base "unfinished changeling" look and that of one specific humanoid; you also get Mimicry and at 3rd level,
alter self). One of the gifts granted doppelganger-like abilities (including
detect thoughts and being able to mimic any other humanoid you can observe).[1] There was a birdfolk heritage in issue #5 of Gate Pass Gazette, the simirengo. The heritage traits included gliding, and the gifts included your choice of talons and darkvision
or full-fledged flight.
So, in Level Up, if you wanted to play a "changeling/aarakocra" using the Mimickers and Simirengo as your parent's heritages, then your choice would be: a PC who can shift between an "unfinished" bird-thing and a single humanoid form plus full flight,
or a bird-folk that can only glide but can assume any number of humanoid forms, or, perhaps, the ability to assume a "were-raptor"-like shape.
So, with how Level Up works, both of those say "part birdman, part shapeshifter" quite well.
And
that is one good way that D&D could do half-species. They won't, because it would involve rewriting all of their races to include something like the heritage/gift divide. But they
could. It's not overpowered. You can have full flight or you can have full shapechanging, but you can't have both. Can you minmax this? Probably. But it would take a
lot of careful reading of all the many, many options for a combination that probably is of fairly limited use. Because min/maxing is usually good for one specific purpose and mediocre at best for everything else. If you really want to cripple your character for one bonus, go ahead.
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[1] The other gifts included shifter-style animalistic-appearance (and at 5th-level, a 1/day wildshape)
or the ability to turn into immobile plants and extrude mobile vines. Because I wanted to get shifters in their as well and plants need more love.