Personally, if we did get a "Player's Guide to Sigil", these are races I could see making an appearance, presuming that more 'common' monstrous PC type races would be better served in a Spelljamming-focused sourcebook...
* New Tiefling variants: We did get Demonic/Abyssal tieflings in one UA, so a refinement of that, and new subraces for Yugoloths and/or Night Hags and/or Succubi makes sense to me.
* New Genasi variants: Although not quite as fluid as the old Elemental Chaos, with the new Elemental Planes line-up, there's still room for Genasi outside of the Earth/Air/Water/Fire quartet. Hell, it's not like we didn't have para' and quasi' genasi in editions past.
* Githzerai: This race is pretty much the only "iconic" and original Planescape PC race we don't have in 5e so far (Genasi appeared in Princes of the Apocalypse, Tieflings in the corebook, Aasimar in Volo's Guide to Monsters). With the awesomeness that is Dak'kon now returned to PC, why not give the option to explore the Children of Zerthimon to players?
* Githyanki: I've honestly never seen the rationale for the entire githyanki race to be so blind to their own hypocricies. I feel that even if they would be a rarity as a whole, in a game where playing "the exotic" is supposed to be normal, why shouldn't "githyanki who realized Vlaakith has enslaved them" be any more unplayable than "drow who's working alongside non-drow" in an FR game?
* Bariaur: These are the third of the original Planescape Races, but I don't honestly know if WoTC could pull them off. Plus, they were always kind of goofy...
* Rogue Modron: The most famous of Planescape's races, after Planetouched and Githzerai. With Nordom now back in the public eye, why not give the chance to let people play his kinsfolk?
* Shadar-Kai: The 5e Great Wheel did retain the Shadowfell, in one of its better decisions, and Shadar-Kai were the "iconic" Shadowfell natives of 4th edition. I can't think of a better "dead realmer" to have walking the planes.