The only Seinfeld reunion ever done (that I know of) was on an episode of "Curb Your Enthusiasm", and it wound up being a pointless exercise (whichis really what CYE usually was, even though it could be pretty funny). I doubt they'll ever do another. Even the Comedians in Cars episode with Julia appears to show some edited-out friction between the two.
IMO a Seinfeld reunion will never be done because of Michael Richards’ misadventure with the “N” word.
Plus a lot of the recurring characters have passed on (Morty Seinfeld, Uncle Leo, Mr. Pitt, etc.)