Geomancer, I reckon.
It's a fairly common and frequent request, it's been outlined as an actual thing within the game world and WHM has been adjusted at higher levels to lose Earth and Wind attacks in favour of Holy themed ones instead. Which, in turn, reduces the overlap that there would be between the two jobs.
Chemist is another common request and might fit a Garlean themed expansion quite well if that is what is next.
I'm fairly certain that we'll be seeing a new healer as there was a lot of backlash in regards to one not being added with Shadowbringers.