Personally, I'd like a real counter tank. Not a little gimmick like what DRK had, but a counter mechanic as a core.
For me, this tank would have several combos and each could end in a special counter stance for a few seconds. If you get hit by an attack corresponding to your counter stance (magical, physical, heavy attacks, whatever...), you'd massively reduce its damage and deal a powerful counterstrike. It's directly inspired by the
Swordmaster of Bravely Default, and, IMO, would have fit nicely for a SAM tank.