Drust are pretty much extinct and bred into the Kul'Tirans, I couldn't see them becoming playable because it'd make KTH redundant
Saberon and Botani are definite maybes now, but there's like... 3 of them on Azeroth. Botani can sidestep it because they reproduce non-standardly, but if we had Azeroth born Saberon they'd basically be an entire tribe of inbreds.
As an aside: with how underwhelming BFA is turning out to be and them starting to go into a mode that feels like damage control, part of me is hoping we'll start getting "In case of emergency break glass" mode and the Devs, wanting to pander to players to keep them playing, will see threads like these and go "Okay what do players want? Sethrak? Okay Sethrak then!" on impulse