Domi signed a one-year, prove-it style deal with the Leafs last summer, taking home 3 million dollars. He had a slow start to the season, but ended up being a very effective player for the Leafs, so he should be getting a reasonable raise on that, and probably some more term as well. The Kraken certainly have the cap space to make the move, but whether or not Domi wants to play in Seattle remains to be seen.
Whether or not the Leafs should re-sign Domi has been a subject of much debate since the season ended. NHL insider Pierre Lebrun believes that he will get between 5 and 6 million dollars a year on his next deal, which has many within the Leafs fanbase balking at the idea of re-signing him. This was discussed on the SDPN podcast.
That being said, LeBrun has also reported that there is mutual interest between Domi and the Leafs to get a deal done. Perhaps he would be willing to take a slightly lower average-annual-value to stay with the team. After all, Domi has been on the record stating how much it means to him to be a Leaf.