I'm not sure one of these things is like the others. I'm all for going after Karlsson. I think RNH for Mantha is swapping positions but getting similar results on the scoresheet. I'm not really enamored with Johnson's game or contract. if we're going after a guy like Johnson, we need to be landing a Karlsson or other top pair D, too. Without fixing that blueline, it's not really going to matter what happens with the forwards.
I'm more ambivilant towards RNH because I expect him to likely be dealt in a year and half when his contract is up and we (hopefully) have a kid or two knocking on the door of that job. He's like a Filppula deluxe to me, and moving Mantha for him would reflect what the Wings actually thought of Mantha. And they'd likely recoup a big chunk of his trade value when RNH is eventually dealt, anyway.
Going off topic a bit, but this is why the Dahlin draft was so big. Win that and we're talking about the blueline entirely differently right now, and going hard after a guy like Johnson looks like a fantastic move to polish off a strong forward group.