I voted trade Wiercioch, but that might not be possible, or at least not get a near equal value in return.
Second option, IMO, would be to trade one of Cowen or Methot, as I personally believe neither is top line quality, and they both can't play 2nd line with Ceci moving forward. Cowen needs to show he's rebounded before proper value could be attained, and Methot would end up being a sign and trade, and I don't like the idea of a sign and trade.
Phillips, Gryba, Borowiecki, and Wiercioch are fine as bottom pairing and depth D. Claesson is ready for some NHL time in case of injury, maybe Wideman in a stretch for bodies.
If Cowen or Methot could be traded for a proper partner for Karlsson, that could fix some of the issues with the Ottawa D. Methot is 6 years and 7 months older than Cowen, yes, but that is a good thing since it increases Cowen's trade value over Methot.
I would trade Cowen straight up, or if the trade needs a value increase, package, and this is where it always goes wrong in trade scenarios, but package Cowen with one of Phillips, Gryba, Borowiecki, or Wiercioch, whichever one the trade partner prefers based on their situation, for a complimentary partner for Karlsson. Different people shudder over this thought because one of their favorite players may be included, or scoff that a player they deem worthless is included. There are reasons to keep all four of the depth defenseman listed, but if we want a top pairing player, value has to be given, and none of them are untouchables, not really.
As a last thought, keeping eight D with the belief that one will get hurt and drop the team to a more appropriate number is borderline morbid. Yes players get hurt, expecting it to happen, soon, just waiting for it to happen, is borderline morbid. How long should the team keep that extra D handy waiting for the injury? And when the injury does happen, do we exclaim, finally?!? Then proclaim isn't it great we were prepared for that, now we don't have to wait for four hours for a player to arrive from Binghamton to sit in the press box. It would require two defensemen to get hurt at the same time to justify carrying eight all the time. BTW, Ceci and Claesson are waiver exempt for the entire '14-'15 season. Wideman is waiver exempt until he appears in 60 NHL games in '14-'15.