I don't see how Karlsson can be part of the Bruins plan, especially if it take Marchand to get him (which I doubt, the Pens are probably looking to dump his contract any way possible). Karlsson will be 35 this year. The Bruins need to be looking to the future, finally, can't see how EK makes any sense regardless of what you think of him as a player. IF you brought him in, that would be with an eye on short term success given his age. If you are trading Marchand for him, further gutting an anemic forward offense - how does this work? Also, this Bruins defense/goaltending has been routinely giving up goals like we have not seen in recent years. It's not like this D can hide Karlsson's dubious defensive abilities.
I don't feel the Bruins have the surrounding pieces to make it worth keeping Marchand unless you lean towards sentimental and loyalty reasons, which is fine btw if not as mercenary as some would like. But if they do move him, it really has to be for something of value for the future. No short term patches. And if you are just getting meh return I'd just keep him. But for real prospects or genuinely useful draft picks, I'd be considering it strongly.
Also, the Bruins and Penguins may be too similar as teams to make for good trade partners. Both teams are fading after long runs of success, both teams are sitting on aging key players and/or who will be free agents in the next 18 months, both teams are short on cap space, both teams don't have great farm systems, neither team is sitting on excess talent on the NHL roster, and neither team has fully thrown in the towel on this season or next (so far). At the moment at least, seems like they would be trying to accomplish the same thing at this deadline. There are many other teams that would be more obvious trade partners.