Good idea as it is. +8 attack is not very hot (we already have +12 and +10 versions) but "partial SoB immunity" is new concept.
Essentially these cards are always +8 or +6 but they must be played as mono deck to enable that "undeniable bonus". They could have special "Suppress bonus" cards that have good ability but enabled ability shuts down their own clan bonus (but if enemy uses SoA on suppressing ability then bonus is again enabled).
Maybe a Revenge Protection Attack bonus... but not a solid Protection Attack. Obviously that would be just wrong. I would be interested to see a clan with a Stop: bonus of some sort... but perhaps that would be too specific against Nightmare and Piranha. Something would have to happen in the game where a major element was added that increased the value of SoB quite a bit before they make a Stop: bonus power of any sort.