I'm going to copy and paste my response to an identical thread on DnD Beyond forums. As people over there can't behave and get their thread locked as a result. It's a potential idea for how hybrid-species could be handled:
--copy and paste starts here--
So assuming a slight rework of the playtest species, where species features are sorted into 'Feature A' and 'Feature B'. (Please don't throw balance issues at me just yet, as this is just using the playtest species as examples). I'm also only using one elven lineage as an example because it's a proof of concept, not a full writeup.
Basically as everything is separated into two columns, you pick one of each to represent a hybrid species. There are two combinations for how each can be mixed, by picking feature A of one and feature B of the other.
Human
Feature A - Resourceful: You gain inspiration whenever you finish a long rest.
Feature B - Skilful: You gain proficiency in one skill of your choice. In addition, you gain one 1st-level feat of your choice.
Elf
Feature A - Fey Ancestry: You have advantage of saving throws you make to avoid or end the charmed condition on yourself. In addition you can finish a long rest in 4 hours, and cannot be put to sleep by magical means.
Feature B - Elven Lineage: Drow - You gain a darkvision range of 60ft. Additionally you know the dancing lights cantrip. At 3rd level you gain the Faerie Fire spell. At 5th level you gain the Darkness spell.
Half Elf Example A
Feature A - Resourceful: You gain inspiration whenever you finish a long rest.
Feature B - Elven Lineage: Drow - You gain a darkvision range of 60ft. Additionally you know the dancing lights cantrip. At 3rd level you gain the Faerie Fire spell. At 5th level you gain the Darkness spell.
Half Elf Example B
Feature A - Fey Ancestry: You have advantage of saving throws you make to avoid or end the charmed condition on yourself. In addition you can finish a long rest in 4 hours, and cannot be put to sleep by magical means.
Feature B - Skilful: You gain proficiency in one skill of your choice. In addition, you gain one 1st-level feat of your choice.