I mean if.... IF Trouba has made it known that he still wants out we essential give up one year of control in a Trouba + for Karlsson swap and the + would have to recognize that he was going to walk UFA at the end of next season as we won't have the cap to sign him and we'll be in the playoffs so we won't be dealing him at the deadline either.
Trouba + Petan + Dano for Karlsson is about the most I would do.
I wouldn't trade our 2018 1st because it would be imperative that we drafted a RHD with top pair potential. (Jett Woo or someone like that hopefully to replace Buff at the end of his contract). It would also mean we'd likely be trying to extend Myers unless Poolman can show he can handle 2nd pairing duties.
We'd be left with the same forward group as we have now with Roslovic, Lemieux, Vasaline, Appleton, etc. still coming.
D would be:
Morrissey - Karlsson
Enstrom - Buff
Kulikov - Myers
Chiarot - Poolman
To me the year of control and the valuable, but unlikely to crack our lineup except as injury replacement forwards, make sense if it leads to 2 deep playoff runs.
If Trouba is open to staying I'd rather just keep him, but if we have to move him eventually I'd prefer to package him up for an upgrade.