Geting all gear slots with primal ancients would be quite an accomplishment. Would indicate a significant amount of time has been spent playing the game. Perhaps such dedication should be rewarded?
How about a Primal Rage buff?
This could simply be a cosmetic effect. Maybe a red aura emanating from you character. Think dragonball z its-over-9000 powerup sort of thing.
And maybe as a token dmg effect, similar to the red soul shard, a periodic burst of aoe damage goes off (but without the dumb going dizzy afterwards).
Feel that getting a character fitted out with all primals deserves some credit.