The two players most likely to move this off season that I believe could improve the Senators are Taylor Hall and RoR.
Recently, and before the deadline, I have seen more than a few articles in the mainstream media about Hall being on the trading block. Many articles further specifying that Hall would best be moved before or during the draft. The reasons for this change in stance in the media (which presumably took it from discussions with Oilers management and sources) is that Hall has become un-coachable due to the constantly shifting bench bosses and isn't showing the same buy-in as other top Oiler players like Eberle, Yak, and RNH who have improved under their new coach. With the obvious holes in the Oilers roster, I concur with the media who believe it is now Hall who is considered the most movable "large" piece of the Oilers.
With that in mind, Ottawa also has many of the assets craved by the Oilers management including young top-4 defensemen (Cowen, Wiercioch, Ceci, etc..) and goalies (Anderson and Lehner). Other pieces nescessary for a Hall deal, including a replacement winger for Hall are also on Ottawa's roster, including Hoffman, Michalek, MacArthur, Lazar, Stone, etc....
As far as value is concerned, my opinion is that any deal should be based around similar players who have been moved in the offseason or before the deadline before, this list includes Kane, Ryan, Spezza, Thornton, Kesler, Heatley, etc..
With that in mind, I would propose something centered to Edmonton around the following: 1 top-6 winger, 1 top-4 defenseman, 1 goalie, 1 pick. The value of each subsequent asset being contingent on the value of the other assets involved and the desires of Oilers management. If there is another team in the NHL that would be able to safely offer more than Ottawa, I'd be shocked.
As far as RoR is concerned, rumors concerning his future status have been ongoing for years and Avs' management's decision to put him on the trade block are well known. Of course, the remaining length of his contract is of significant concern, especially in a City like Ottawa that is not a big draw for Free Agents.
If RoR is not unreasonable with his contract demands and team selection (something the Avanalche would know in advance), the Avalanche could significantly improve their haul for RoR (and specifically Ottawa's interest) if they allowed teams to speak to RoR's agent to determine his willingness to sign an extension. Now, obviously this is a pretty big gambit for the Avalanche but if circumstances are right, this could benefit all concerned parties.
As with Edmonton, Ottawa also possesses many of the pieces that Avs would be seeking for RoR, including young upcoming NHL top-4 defensemen, picks, prospects, and replacement wingers.
In my opinion, both RoR with his two-way play and fantastic discipline, as well as Taylor Hall with his drive and creativityare capable of providing a long term solution at the Leftwing for Ottawa and both are considered movable. Further, I don't believe any current free agent would provide a similar improvement.