Most of the spells I made were focused on the respective Job's roles and their weaknesses. For example, WHM is known more for its burst AoE heals, consistent heals and burst damage; I made Cure IV, Renew, Stone III and Banish in that vein. However, WHM is noticeably weak when it comes to high-end support and utility; it is very much a healer bot. That was my inspiration for Drown and Arise; the first being a DoT that will always hit (acc problem solved for one spell), and its major effect of increasing DPS for its duration. Imagine in a raid, with the two tanks and four DPS attacking for the duration of Drown. It would average to 6 GCD attacks each; 36 attacks in total, not counting off-GCD attacks and skill/spell speed. 36 attacks, at 10 potency each. Added with Drown's own potency, it would result in a total potency of 385. If there were more attacks, for example, procs, bloodletter, the other healer casting spells, Drown's effectiveness increases.
Arise, similarly, plays on WHM's strength of big heals and spells, but also bringing usefulness to both progression and farming. Progression to instantly bring that important offtank/healer/highest DPS back up fast, and farming to almost mitigate a mistake (that caused a death) by bringing the (usually) DPS back into the fray straight away.
For SCH, their biggest strength is their damage mitigation, high burst single target heals, and their utility and DPS. Their single target heal is so good, actually, since Lustrate scales with tanks so well in addition to the fairy, that I didn't think they needed an additional single target heal. What I played on was their strength in utility, and their weakness in all forms of AoE. I heaped on more utility. DPS get a benefit from Imperil, for that evasion down; it even benefits the healers to land their spells more. Support skills were truly to support other; Accession makes you and your other healer or your fairy super awesome for that duration. Imagine a double-effect Protect, or Fey Covenant, or Medica II/Whispering Dawn, or double effect Divine Seal/Fey Illumination, or even a quadruple-barrier crit Adlo. Penury also supports either your healer or your caster DPS; keeping your BLM in Astral Fire stance for longer.
SCH's weakness, I fixed with Elutriate, while also combining it with SCH's unique concept (percentage of health). For AoE DPS, however, I had to incorporate it into the ACN abilities, because SCH abilities are always enhancement/heal based. So I had to think of an AoE DPS skill that did not destroy SMN, or destroy ACN's concept of low-mana cost spells, low potency burst, but good sustain. Aurorastorm was what I thought of; making the two only damage spells AoE, that were not overpowered or expensive, AoE. Again, playing on SCH's strength of good mana retention and their concept of low-burst damage.
One thing I haven't done is capitalise on Aetherflow for SCH, aside from Aurorastorm. It's hard because SCH relies on Lustrate for big heals, and that uses Aetherflow, that to think of something useful that also uses Aetherflow will be hard.
I may think of more abilities. I didn't realise how fun it was until I started thinking of stuff that worked.