Rereading the thread, How about Willpower being used as Armor versus spells? How about just Harm ones?
Tap self: Can not be used to heal. Question: Can is be used to boost Spell Heal? Can it boost Arcane focus?
If one were to go this way...I would say resistance for harm spells only because semantically it seems that Willpower would be used against spells that effect the mind more....Of course if you think of it as the will to keep going I can see that as well...Argued myself in and right back out.....
This is what I decided to use:
Resilience = Resistance + Endurance + Willpower. This removes Arcanum from the Ability Tree but adds Endurance. Otherwise, Arcanists would have an unfair advantage when using the "Tap Resilience" option.
You can also continue to allow an Arcanist's Specializations and Masteries to serve as separate "active" Defense Pools against magic of the same source and school, but at 1/2 MV. Do not add the MV of basic Arcanum. For example, a wizard with Arcanum D10 > Mystic D8 > Wizard D6 would have an extra 4 points DP against Mystic spells, or 7 points versus Wizard spells specifically. Apply these defenses before passive Resilience DP.
For creatures less than full-fledged, I just left it alone. The real problems occurred when folks made the RS 1/2 HP rather than 1/2 primary threat rank. With the new rule allowing the tapping of RS score, overcoming RS for spellcasters, along with Eldritch Focus Advantage, should not be a huge problem.