Add Shell, and don't make it stackable with protect.

[Aldora]

But only White Mages had Proshell, so Scholars just had to deal with only having their shields to mitigate magic damage. If they want any group composition to be viable, passive mitigation buffs either need to be available to every healer or not exist.

And that's why Protect has been changed to a Role Spell/Ability which does the same thing for everyone. All 3 healers now have access to Protect, which will increase both physical and magic defense.

But, let me ask you this. How does a scholar (or Astrologian) become less viable in a group composition, just because it didn't have a "Proshell"-trait? The only "shielding" ability the White Mage had was Protect and Stoneskin. The White Mage always was the burst healer and the Scholar always had its shields (which gave a shield for the amount of healing done) and fairy. They both had their own identity in ARR and complemented each other well during raids.

Then the Astrologian was introduced, which was meant as an alternative to the White Mage and Scholar combination for Raiding. The flexibiity and Card system was the strength of the Astrologian and gave it its own identity.

It's not necessary for all 3 healer classes to have the same "passive mitigation"-buffs. It doesn't really matter for the regular dungeons (due to their more casual difficulty) and with raids, you could get 2 healers that complemented each other. So, that difference that the "Proshell"-trait gave you was only marginal compared to what the spells from the other classes could give.

But, with my previous comment, i was only referring to the fact that Shell was a separate spell in 1.0 they merged with Protect and offered as a trait in ARR. The Proshell trait was later removed and the effect permanently incorporated within the Protect spell. What the OP was asking for was a situation we already had in 1.0, but the team stepped away from that. They even considered removing Protect entirely with the release of Stormblood. So, we won't be seeing Shell in the game anymore.