Karlsson with Hughes would be a disaster. Hughes needs someone that can cover for him when he pinches for offense, not someone who is going to double down on that offence and be absolutely woeful when trying to defend or cover for his D partner.But this whole having 5 forwards on the ice idea is precisely what the other teams thought when the Sharks acquired him. Certainly, Karlsson and Burns had their moments as a dynamic dual and they did make a run in Karlsson's first year. Don't get me wrong, prime Karlsson was something to behold. Then again, prime OEL was certainly a strong contender for being a top 5 Dman.
If Penguins retain, there isn't a whole lot of risk to Karlsson. He would only have 2 more years left after this season. I think Karlsson can still put up points if he can stay healthy and you let him play his game. If you want him to be a systems defender and expect him to defend well he's not your guy. I'm not sure that he would be a fit alongside Hughes.
Karlsson's defending is that bad, it's not worth the offensive value he brings at this stage in his career.