Horwath
Legend
Have all racial/species features a default cost in PHB.It's not a bad system per se. But it's going to require a hefty chunk of real estate to implement. Every species in the game will have to be broken down into primary/secondary/tertiary lists. Weight will have to be given to each ability so determine what is considered which type. You would have to do this for all nine PHB species, and realistically for the 30 more in MotM and possibly the dozens more in various supplements, some of which haven't been updated to the new format yet. Then you'd have to look for the broken combos and superior options that clamp down on them. (For example, a hybrid changeling might only be able to shapechange x times per day rather than at will).
Is all that doable? Absolutely. But to be done right, it's going to cost a lot of R&D, playtest and space in the book. I don't think that is feasible for the 24 PHB. That would be awesome in a 25 supplement book though. Markov's Guide to Genetics or something. I just don't want to see such a system rushed out the door in order to have a PHB half-elf replacement. There are too many variables that can break when combining things that weren't designed to be interchangeable.
And have all races/species same budget.
then when you create a hybrid of two, you can pick from both lists, but you need to stay withing budget.
If you have trouble filling the budget, we can always have some cheap "cultural" features as bonus tools, languages or weapons. Maybe one extra free skill.