This is what the adaptive armor mastery should have been.

adaptive defence

gain 1% of your bonus health, as bonus armor or bonus magic resist. Which ever one is lower.

typically late gane tanks would have maybe 4k max hp, with about 2k being bonus hp, 1% of this is 20, 1 armor or MR per 100 bonus hp.

or 0.75%

which would be 15 armor or MR at 2k bonus hp.