The end boss of e12 was speculated to almost certainly be Ultimecia and many were convinced the 4th healer would be Chemist, with strong "hints" for both ingame. In Stormblood we thought we were getting dancer, but it turned out to be nothing to do with it and in Shadowbringers we thought we were literally becoming the Warrior of Darkness and going down the dark/corrupted path yet nothing of the sort happened.

Enix like to go out of their way to avoid the popular assumptions and "surprise" us with something different. For the healer they even went out of their way to create an entirely new class with a completely unique weapon.
I'm almost certain it won't be Reaper or Necromancer. It will be something no one expected. Possibly even another entirely new class unique to FF14.