While the Seattle Sounders can add a Designated Player in the upcoming transfer window, they are also looking towards next winter. After the 2017 season the team holds an option on Clint Dempsey’s contract according to Sam Stejskal at MLSSoccer.com’s conversation with President of Soccer Garth Lagerwey.
Stejskal also relays that Dempsey will either be a Seattle Sounders DP in 2018, or a Free Agent. If the Sounders were to decline Dempsey’s option, he’d immediately become the highest-profile free agent in MLS history.
Assuming he's fit and playing anything like he has been, though, it seems a reasonably safe bet that the Sounders will want to bring Dempsey back.