Cards are banned for a reason. Though they may seem "not so powerful" by themselves, they're powerful in combination.
Arno -- it's not because he's overpowered by himself, but soa + dr + all the other soa dr's in the clan = problems
Arkn -- pill 2-3, or take 4 damage to the face. also has great synergy with Wardom.
Hawk -- overpowered by himself. even more overpowered with his great low star cast
tl;dr, no. cards are banned for a reason.
A great idea and I don't think having only ONE permanently banned card, unbanned for a week, is asking for too much trouble. It would mix things up and make elo more interesting at the very least, nothing but good news. The way they could manage it would be to, along with the weekly votes for cards they want banned for the coming week, players could vote for the card that they wanted unbanned temporarily.
I'd kill to see a Kiki Cr slip through though.
Anyhow, you can't do things just for the sake of shaking things up. There's too many problems with that. The character that's unbanned may lead to an overabundance of said clan and many similar decks. For instance, if you unbanned Lehane or Hawk you'd see mostly Sentinals staples all week. So, in essence, although you're wanting to unban characters for variety and fun you're really taking out variety and the fun of it.
When an OP character in a strong clan or sometimes even a weak clan you either have to conform and abuse the op unban for the week or dedicate your deck to fighting that character. Take Spyke or Kalindra for example. Totally op cards and when Kalindra is unbanned or when Spyke was unbanned, how often do you see their respective clans used in comparison to when they are banned. The answer is a lot more. It unbalances the meta game and makes it centric to a specific card.
Plus there's the logistics of it. How do you decide which character gets unbanned? How do you handle the DT penalties of the unbanned character?
^ also not necessarily, unbanning a perma ban would bring back that clan into most games, but would also allow for people making presets that beat that ... bad explanation really... like if Arno was unbanned, Roots come back, so people could pop out the stop: cards and have a bit more fun, this would increase the use of cans like Jungo perhaps, so it wouldnt just increase one clan to full use, it adds fun and flavor to a lot of boredom