I think the Bucks or Raptors should go all in and acquire Leonard (for different reasons). The East is now more open and both of those teams have an incentive to try and capitalize on the present. For the Raptors it gives them one last hurrah. Add Leonard to DeRozan and you have a dynamite wing combo. Not sure if something like this works for the Spurs, but Powell, Poeltl, and Anunoby I believe gets them enough salary. It would strip the Raptors young guys, but would give them an incredible starting 5 in Lowry, DeRozan, Leonard, Ibaka, and Valanciunas. Maybe you can convince Leonard to stay with that sort of team going forward, especially if they make the finals.
The Bucks on the other hand could use a deep run and a vet to help guide Giannis' ascension. And who knows maybe the thought of playing next to Giannis for the next decade sounds appealing to Leonard. A trade centered around Middleton with maybe Brogdon and/or Maker may be appealing to the Spurs. The Bucks also have enough salary that they can swap contracts so like Henson and Mills or Gasol can be added (Spurs shave a year).
Both Milwaukee and Toronto could use a Finals run to really energize their fan bases and Leonard is clearly available and with no one giving up primo assets, it might be the time for the Bucks or Raptors to engage discussions.