Glitchling, for sure, I think. The second try at the Ardling felt like it was pulling in a more Planescape-y direction, it didn't get PHB numbers but Crawford said they were positively received and we would see then again, seems a good place.What races do you think will appear in the 5e Planescape book?
Rogue modrons (glitchlings), maybe some revisits of Tiefling and Aasimar... who else?
They may or may not reprint anything from MotM: just saying "anything from MotM would work!" may be fine, but reprints of several options could happen.
They may also throw in some stuff they feel should be there, but I'm not sure how likely that is.