If you trade for Karlsson then you have to go out and find him a Methot. Not Sanderson because he needs to carry his own pairing, it'd be a waste to turn him into Karlsson's clean-up guy (especially when Sanderson is the better player at the moment).
It's just completely unrealistic, and doesn't make any sense to bring him back. Not to mention, Karlsson hasn't exactly shown to be a reliable player if he isn't the #1 dman on his team. This is a player that needs to control the puck in order for him to be special. If you expect to just slot him in as our #2 guy behind Jake Sanderson then it's not going to go well, he doesn't do well in that role.
The only chance of Karlsson ever coming back is when his deal is up and he's already adjusted his playstyle to be a bit more of a stable supporting piece rather than a franchise player you build your team around. I imagine Karlsson will be 100% interested in coming back to Ottawa when his deal is up if he can still play (he would probably take a hometown discount too), but the question would be does Ottawa want him?
