Imo Vegas is the only possible destination if it is true the Bobby Ryan's deal is attached.
Bobby Ryan is not a LTIR candidate. He is playing badly but he is playing. He's only 31.
Ryan has a cap hit of 7.25 AAV for 4 more seasons. Ottawa is already retaining on Phaneuf, so I doubt they would retain on Ryan considering you are only allowed to retain on two deals at once, so that would paralyze them on retention for a long time. (edit: I was wrong about two retentions. You can have 3. So maybe this is an option, although it leaves Ottawa paying a lot of cash in retention deals, which defeats their purpose a bit)
Karlsson has 6.5 AAV, but it is about to be 12 AAV in one more year, and that's conservative considering he said he's not interested in a discount of any sort.
Any team trading for Karlsson could send salary the other way, but the whole point of this trade is for Ottawa to shed salary, so it would still be very lopsided in terms of cap leaving Ottawa.
Ryan has a NMC. He would need to be protected in the Seattle expansion. No team would find this acceptable except Vegas, since they don't have to participate in the Seattle expansion.
With the cap expected to see the highest single season rise in a very long time, multiple teams are in a position where they'll have added cap flexibility.
There's probably at least a half dozen teams who if they really wanted to could make Karlsson+Ryan within the constraints of their current cap structure.
Ryan's NMC only relates to being assigned to the minors. There are no expansion draft considerations for Ryan. The Senators were able to leave Ryan exposed to Vegas WITHOUT him having to waive his NMC. The same will hold true for the Seattle draft.
There's a high probability of a lockout in two seasons. This means there might be compliance buyouts. In a best case scenario, a team acquiring Ryan could hope to only have to grind through a single season with Karlsson on his raised AAV and Ryan at 7.25M before being able to use a compliance buyout to rid them of the final two seasons of Ryan's contract.
I don't expect an amazing return for Karlsson with Ryan bundled in. I would expect something in the realm of a young NHL player, a top prospect, and two 1st round picks. For Karlsson in a trade, that's a bargain.
Vegas was likely the only team who could swing that at the deadline, but during the off season there should be a few more teams who have the cap flexibility to get in on Karlsson+Ryan if they wish to do so.