I like this but it means that essentially you are letting Seguin go, so if Dallas makes that choice next year this could certainly happen.
I think that is a solid trade btw if you are Ottawa how do you say no.

Solid trades and signing. Just wondering what the long-term outlook is for signing Karlsson and Seguin?

Impossible. They still have to re-sign Seguin, plus Nichushkin isn't even on here and will make roughly 3M for the next 2-years.

These are the solutions:
Seguin will take 5.75M$+ Anti Niemi buyout 1.5M$+Jason Spezza Retained salary 3.75M$=11M$ 8 years
Karlsson will take 6.5M$+Kari Lehtonen 1M$(keeping 0.5M$ for the backup goalie)+ Salary Cap increase (lets say 2M$)=9.5M$ 8 years.
Dallas finds a way to trade Stephen John,Mattias Janmark and Radek Faska next year on Trade deadline or next summer for something cheaper or draft picks and we are good to go.
Marc Methot will go on free agent (4.9M$ free cap space to add to Esa Lindell and for others).
Yh Forgot about Nichushkin. (Maybe trade Devin Shore for picks).

So you're essentially selling off the whole team for 3 guys? Gotcha. Even with Karlsson, our defense gets massively worse. You're getting rid of Heiskanen, Honka, AND Johns for Karlsson, not to mention losing two of our best young forwards (Faksa & Janmark). If I'm Dallas there's absolutely zero chance I go for both of those guys. Go after Tavares and leave the defense alone. Dallas had the 7th best defense in the NHL last season. No need to drastically overhaul it. Even with Tavares, the Stars can still re-sign Seguin and will have plenty of cap to re-sign Janmark, Faksa, Honka, Lindell, etc. when their contracts are up. Karlsson is definitely a great player, but considering all the pieces we'd be losing he's not worth it.

Eeeeesh. That's a whole lot to sell off just for Karlsson and Tavares. I'm all for getting rid of Hanzal and Spezza, though. To me, I don't trade for Karlsson because the price is going to be much too steep (regardless of the team he gets moved to) for my liking. I try and sell the core of our team to Tavares and see if he takes approximately $9m per season. With the expiring contracts up next season and the increased cap space, should be enough to re-sign Seguin on top of that.

With how much you'd have to sacrifice via trade to get Karlsson, I much prefer Benn, Radulov, Seguin, Tavares and Heiskanen over only Benn, Radulov, Tavares and Karlsson.