If we're talking about UFA defenceman who are realistic targets for us yet will still potentially make an impact, I'd say Joni Pitkanen* is our best bet.
I know I put Niskanen in as my UFA defenceman because he's young and good and could be a long-term piece.... but he's having a career season, the kind that I think will be hard for him to duplicate on another team, and as such I think he'll be overpaid as a UFA. I like the player, but I'm not crazy about the contract someone will sign him to... he'll get overpaid by at least $2mil/year for the production you'll get out of him going forward.
Pitkanen seems like he's right in our wheel-house: he might sign on a discount because he spent the entire year on the shelf with injury, he's not so old (he's only 30) that his play is going to start rapidly declining, plays a great transition game, is a fantastic skater, great offensive instincts and would be an amazing compliment to Karlsson on the powerplay. He's not a physical guy, but we have a ton of those right now (Methot, Cowen, Gryba, "Phillips Sometimes")
If we could get him on a 2 year, $6-to-7 mil deal... that would give our blueline the real kick in the pants it needs. Ideally we need a shutdown guy, but there aren't any shutdown defencemen on the UFA market this summer that don't come with some pretty glaring warts.
* assuming Pitkanen doesn't show lingering effects from his heel injury - the last thing we need here is a Ryan Whitney type of guy, whose career is cut short due to injury.