Outlander already is a background, but Berserker could have, and I agree should have, 100% been a fighter subclass. But that ship has sailed.
What ties the existing Barbarian subclasses together is they fight better in an altered state. It might come from battle frenzy, divine gift, elemental affinity, or Ghost power (which all together is indeed a silly basis for a class) but Giant Might is too close to being Rage with Large size tacked on. It (and therefore also Great stature and Blessing of the All-Father) muddies the waters far too much for my taste.
If the subclass is about Runes it should be about Runes, but if it's about Giant powers granting you extra damage, Advantage on Strength stuff, and resistance to damage all for one minute then you've got yourself a Giant powered Rage.