I was thinking how the custom lineage from Tasha, with a few tweaks, could work as a catch-all. Give them three choice points:
Darkvision or a skill
A ribbon ability (free tool prof, hold breath, climb speed, fey ancestry)
A larger ability (stonecunning, free spells, or a feat)
Flavor and fluff to taste.
Use the half races of old as examples.
It wouldn't be robust enough to make an aarakroca-dragonborn, but it could cover a lot of generic mixes and things not covered directly in the rules.
Pick your racial traits;
you have 6 trait points:
skill proficiency, max 2 times
combination of 3 tools, languages and/or weapons, max 2 times
expertise in one racial tool or skill
darkvision 60ft
darkvision 120ft, worth 2 points
hardy: +2 HP, +1 HP for every level after 1st, worth 2 pts
damage type resistance(except bludgeoning, slashing or piercing), max 2 times
healthy: advantage and resistance vs poison and disease
durable: advantage on one saving throw
+5 move speed, max 2 times
climb speed equal to move speed
swim speed equal to move speed and hold breath 1hr
burrow speed, half your speed
fly speed(wings), worth 4 points
cantrip, max 2 times
one level 1 spell known, cast for free once per long rest
one level 2 spell known, cast for free once per long rest
one level 3 spell known, cast for free once per long rest, worth 2 points
extra feat: worth 5 points
natural armor 13+dex or +1 AC when wearing armor you are proficient
natural weapon 1d6+str or 1d4+dex, one attack as bonus action
large frame: count as one size larger for carry capacity, grappling and similar
negative traits:
slow: -5ft speed, gain one trait point
small size: gain one trait point
sunlight sensitivity: gain one trait point
-1 to one ability; gain one trait point, max 2 times, cannot be taken on ability 8 or lower.