I'd surrender something like Gagner for him, as Gagner can at least play wing in a top six role.
Spezza will likely bounce back, so my convoluted and contrived scenario would be if they needed immediate cap space and didn't want to attach futures back.
Failing that....does Dallas need top 4 D? Vancouver has some good top six wingers and good 2nd pairing D to choose from, would Spezza +futures for some combination of Baertschi, Tanev and something be in the right ballpark?
Dallas already seems to have their Top 7 locked in. I'm not going to argue that Tanev wouldn't be an upgrade at some spots, but on the right side, he's not pushing Klingberg, and Dallas seems pretty committed to Johns and Honka in terms of let them play so they have a chance to develop this year. On the left side, unless they trade Methot, they're going to go with him and the other Finns (Lindell and Heiskanen).
Delaying either of those 4 guys development isn't worth a potential slight upgrade on defense. The better risk is to just take the chance that Spezza bounces back from a career low shooting percentage and a terrible relationship with Hitchcock. The dude has slowed done, but Montgomery is saying the right things. He said Dallas will be structured when they don't have the puck, but he wants to let the horses run when they do have it. My guess is Spezza could see a reunion with Janmark, and this is just a guess, but I think Montgomery puts Radulov on the 2nd line. Dallas suffered by being a 1 line team. Dallas' #1 priority this year should be a scoring winger. If it was a LW, even better. If Spezza had two horses next to him, I think he could be a guy who can be the good 3rd guy on that line. Janmark has played RW and looked fine. He and Nichushkin could a shot on Benn and Seguin's win. Those two have traditionally carried the other guy. I think Janmark is a guy that can do more than be carried.