While we don't need Karlsson, could you imagine a top 4 of Josi/Karlsson and Subban/Ekholm?
My guess is Ottawa would want a cost controlled Ellis, Tolvanen, 2018 1st and 2019 1st. Now while that seems like an overpayment is it? Karlsson is an elite defenseman, no other way to put it. He is an upgrade on Ellis. Heck, he's probably an upgrade on Josi and Subban as well. Giving up Tolvanen and the two firsts would sting but once again, we're giving up one NHL player in this deal. Tolvanen, for as good as he looks, could be a flop. We have no idea what we have in the kid. I'd love to be optimistic and think he'll turn in to an elite sniper. Heck, even a solid top 6 forward would be great but no one knows what he'll look like on the smaller NA ice. The two 1st's will be late in the first so more than likely, not game changers.
So for possibly the best defenseman in the world, we'd be giving up a very good if not great first pairing guy in Ellis and three pieces that we have no idea how they'll turn out. Does it possibly gut our future? Sure. Does it give us the weapons to compete for the next bunch of years. Sure does. Thing is, while Karlsson has said he's going to get paid, would he take a little less to stay on a team like this and have a shot at a bunch of Cups? That is the question Poile has to ask and if he feels like he'd re-sign in Nashville, he has to think about this.
Down side to this is messing with the chemistry of the team. That is why Poile is a GM and I'm sitting here typing this. Could put us over the top or it could F ever thing up.