What races do you think will appear in the 5e Planescape book?
Rogue modrons (glitchlings), maybe some revisits of Tiefling and Aasimar... who else?
Glitchlings are either just a stealth test for rogue modrons or are effectively replacing them as a PC option - same general idea, but with a more free-form body type and potentially not locked in as kill-on-sight for all non-rogue modrons encountered.
Doubt we'll get any updates to tiefling since they are getting an update in the core rules revision next year. Similarly, I doubt ardlings will make it in - if anything they seem to need more work pinning down their central conceit, and I don't know if they had the time necessary to hash that out between deciding to drop them from the '24 PHB and Planescape needing to be finalized for print.
Also don't see much chance for an update to aasimar, since they were recently updated back in Monsters of the Multiverse. Likewise with the genasi, githyanki, githzerai, shadar-kai, and all the existing Feywild-centric races (changeling, eladrin, fairy, etc.).
That basically leaves the bariaur - which could be pretty easily derived from the centaur and satyr stats and thus may not need a playtest - and some of the more obscure/less popular planar races like chaonds, zenythri, paragenasi, etc.
So, most likely just glitchling and maybe an off chance of bariaur.