Personally I’d do the following:

WHM:
Add Water/Banish as a 2.5 second cast time spell, with a 20 second cooldown that later gains charges.
Add Thundercloud system to Aero.
Readd Aero III
Readd Cleric stance, but reworked to boost damage and healing actions, later upgrades to Plenary Indulgence and gives access to a use of Quake, Tornado and Flood that replaces Stone/Aero/Water respectively.

SCH:
Add Miasma, Miasma II, Miasma III as spells (Bio III as well), which later upgrade to Toxify when Biolysis is available. But give it a 21 second timer on it instead of a 30 second one.
Make a new DoT that works like old Miasma II, I’d call it Scan/Intel personally.
Bane gets added, either as an upgrade to Energy Drain or Becomes its own thing.
If Bane is it down thing made Energy Syphon an upgrade to energy drain.

AST:
Bring back Stella but have it work like the previously mentioned Water/Banish but as an instant cast. Genuinely I think AST can get away with so little because of the card system.

SGE:
Go crazy, make it have a bunch of actions that both heals and attacks, give it a 1-2-3 combo, either like PCT or like other jobs, give it a Dot that has a higher potency but is way shorter, or a dot that lasts longer but each tick is weaker. It’s the so called DPS healer like how GNB is a DPS tank, go crazy, make it something interesting