Well, obviously they'd have to be very careful with what abilities they printed. Falkenstein's would be much too strong if doubled. But what about something like this: 6/3 with "-1 opposing damage, minimum 0"? Or a 7/1 with "Damage +3"? The idea would be to have abilities which, by themselves, are marginally less useful than what you typically see with other clans but, when duplicated, are marginally better.
@Thoazol: I think the trick is that their abilities would have to be about 75% the strength of an ability of a card in a typical clan. That way, when both copies of the ability are active, they can sometimes have an upper hand, but when only one copy of an ability is active, they're weaker than Skeelz.
Copy Opp. Ability would be ridiculously hard to balance, so I wouldn't expect to see it, ever.
As far as clan bonuses we haven't seen yet, I could see a clan having Defeat: +2 Life or Heal 2 as a bonus, but I think both would have to be balanced with maximums, e.g. Defeat: +2 life, max 16, or Heal 2, max 11.
I would also like to see a drain life/drain pills ability/bonus-- that is, a successful attack drains X life/pillz from the opponent and adds the same to you. X in this case probably would be 1 for a bonus, and not more than 2 for an ability.