From what they've insinuated by the recent "subraces" conversation, i.e. subraces could be applied via themes or backgrounds. It is entirely possible that half-races might be applied thus, as well.
I'm, not saying or advocating they not be there...and I highly think they will (and should: half-elves and half-orcs, at least, have a loooong standing place in the D&D historical record). But, just as a potential different road, which again allows inclusion or not, as the DM desires, they could conceivably do it with themes and backgrounds (now, whether these will be separate/additional "Racial Themes/BGs" we don't know...I don't think. But that is also not so far/terrible a possibility either.)
So you could have an Elf character...gets ABC special abilities. This would, presumably [to my mind] be a "High Elf" character.
Apply the "Wood Elf" theme/BG: swap out and/or add special abilities GHI.
Apply the 'Half-Elf" theme/BG: swap out ABC and/or add special abilities XYZ
Or, choose a Human character...apply the Half-Elf theme/BG with the same bells and whistles (or slightly adjusted ones since the "emphasis/base -presumably dominant- race is Human instead of Elf?)
Same for "Human" race with a 'Half-Orc" theme/BG ...special abilities KLM.
Honestly, as I'm thinking about it...Tieflings and Aasimar could be supplied this way, as well....and then, no doubt down the road in expansion material, your "half-giants" and "half-dragons" and "half-elementals" and whatever else you want. I seem to recall, waaay back in 2e, someone playing a "half-medusa" (male!, as I recall). But, yeah, I would imagine, once 5e gets rolling and the supplemental/expansion/additional material comes out, you'll see any and everything being plausible through the themes and backgrounds.
Someone mentioned, upthread, using "templates". I understand/have read lots of disappointment/disagreement on the implementation of "templates" as they once existed...but essentially, with Themes and Backgrounds, that is what the new game seems to be very well constructed for. It's just that instead of apply a single "template" you have the assumed width and breadth of other themes and backgrounds that still make your character, inherently, different (in at least some ways/abilities) than anyone else.
Also, depending on how they categorize/list/present the racial abilities, you could conceivably have one Half-Elf character with, say, "30' low light vision, 30% resistance to charm magic, +3 to Perception ("Elvin Senses", let's say just for the sake of example). The other Half-Elf character in the group might have/choose: "30' low light vision, X% move silently in woodlands, immunity to ghoul paralysis" (again, just guessing/making up possible racial abilities that could be chosen)...might be too much, too fiddly, but I kinda like the idea that taking a certain racial BG doesn't guarantee you the exact same abilities...particularly for half-races.
Just kinda rambling now, aren't I? hahaha.
Happy Friday all.
--SD