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Yaarel I’m unconvinced. Those feats are among the least worthwhile feats in the game. They should have been folded into other feats, with no loss to the other feat.
the fact that they wildly overvalued those feats isn’t going to impact how I view a new option.
Especially because of the gaming design of bounded accuracy, both the attack roll bonuses to hit and the AC bonuses to avoid being hit, are unusually valuable in 5e compared to previous editions.
Officially, in order to wear chain armor with proficiency, one must invest in two half-feats, light and medium. Two.
Even if a chain shirt was considered light armor (as in 3e), it would still be best light armor possible. Again, towards a half feat.
• Mage Armor AC 13 + 5 Dexterity = Full Plate Armor AC 18
Because Dexterity Fighters are already at an advantage over Strength Fighters, Fighters and other classes already tend to dump Strength and improve Dexterity. So, the high Dexterity is already in place. There is no special investment. Lots of Dexterity Fighter optimizers would love to have Mage Armor for free.
Mage Armor = Plate Armor
AC 18
This 18 remains balanced, but bounded accuracy requires caution concerning AC.
The benefits of investing in Strength also become uncertain.
In the 4e clone thread, a Fighting Style swaps the proficiencies of both medium armor and heavy armor to gain Mage Armor. (This is intended for Eldritch Knight at level 1.) If one argues giving up two is too much, and only needs to lose heavy armor to gain Mage Armor, we are still in the ballpark of a half-feat.
I feel,
• the value of light armor plus a shield is about equal to medium armor plus heavy armor
• the value of a half-feat is roughly four skills.
I estimate,
• light armor (4) + shield (2) ≈ medium armor (3) + heavy armor (3)
Note, the shield would be worth 4-ish in terms of AC, but the opportunity cost of occupying the offhand mitigates it.
With these numbers in mind, proficiency with all armors (4 + 3 + 3) would total 10 points, thus be an appealing feat, compared to a standard feat (8).