It's a complicated question.
As a budget team, do the Senators go with the Nashville model and build around an all-world defense of Chabot-Karl/Dahlin-Ceci/(Insert two of Jaros, Claesson, Englund, Harpur, etc).
Or do the Senators capitalize on Karlsson's value, trade him for a franchise C or a package of young players?
With any team, it all comes down to cap, with the Senators the problem is exasperated due to their (current) 68M budget. If we draft Dahlin, without a doubt the floor for him will be something similar to Ekblad which is a 7.5M x 8 years after his ELC. Should we set ourselves up for a situation where we are paying Dahlin and Karlsson a combined salary of as much as 20 million? Once we sign Karlsson, he'll most likely have a full NMC, so there's no sign Karlsson now, trade him when Dahlin's ELC is up.
On the flipside, Phaneuf's deal expires the same year Dahlin's ELC would kick in, while Ryan's deal expires the year after that. So it's plausible the team could pay both Dahlin and Karlsson.
The question basically becomes, what is more likely to give us a cup, going all in while Dahlin is on his ELC by keeping Karlsson, or trading Karlsson for a massive package of young cost controlled talent and hoping to re-tool and make the team more competitive a few seasons down the line at the expense of winning now.
The odds of us getting Dahlin are incredibly low, but I guess it's fun to discuss.