It would be nice to see something entirely different.
perhaps a clan that has -2 self damage, which would Explode all over Nightmare
so, cards would have slightly higher than normal damage, but it would be reduced because of their bonus.
(unfortunately, these cards would slip into non related decks like Pussycats
mono decks, would suddenly have nasty 6/5 2 stars)
you could also try the same for power, or maybe have a courage -4 damage min 1, which would have some game changing properties.
Courage Poison 2 min 1
<- courage bonuses are too easy to stop anyway, so it wouldn't be so game breaking
or, we could go with no bonus, just good abilities.
there are a lot of clan bonuses you could take on.