IMO, the solution to the Aegis is the solution to the lack of Mastery options for Ritual Casters besides taking Mercurian Magic, a strong Gold Cord and Cautious Sorcerer.
All these are useful to any wizard that'll cast spells "on the field", who also benefit from cool mastery options. But a Ritual Caster never casts on the field, and has little Mastery Options. He can't focus on his signature spell like other magi.
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In Andorra, we have a few masteries that are "Ritual-Only", something which, IMO, is sorely lacking in the RAW.
One of these is the Secure Ritual Mastery, which doubles the botch reduction from your Mastery (And I would see it being expandable by multiple choices).
Another HR I would implement is that wizard's communion doesn't impact Mercurian Magic.
Take a Mercurian Caster who tries to cast a lvl 20 aegis, with 4 other magi.
Assuming a gold score of 1, and a Mastery of 3, that leaves only 1 botch die. Which is manageable, and rewards the Ritual-Focused player, just as the Flambeau who focuses on PoF is rewarded by his focus.
A non-mercurian would occur 10 botch dices (14 for the lvl 40 aegis) and, in the same situation, would need a higher Mastery.
The Fixer, David Chart has addressed the issue of casting rituals, and Mastery, here . And, so, with that understanding, one never need learn mastery greater than one for rituals, especially the Aegis, which will be cast during relaxed conditions while mastered. So, having additional mastery options for Ritual Magic isn't strictly necessary from the perspective of reducing botch dice for casting.