Larry David wrote the one hour season finale entitled The Pilot which aired, I believe in the spring of 1993. This episode featured many cameos by characters from previous episodes. I think this would be the possible style for the Finale. An episode rife with obscure and famous references to the Seinfeld world would be a lot of fun for avid viewers who thrive on the trivia and minutiae of the show.
Don't look for any long lost relatives or anything new at all. Look for recurring themes and running jokes that we have loved and which have become part of our lexicon.
Here's what I would love to see...
Babu Bhatt returns from Pakistan to exact his revenge on the very bad man, Jerry, but of course is thwarted and humiliated anew by Jerry "Even-Steven" Seinfeld.
Morty and Helen Seinfeld and Frank and Estelle Costanza...they have really been the glue that have held the show together through some lean plots...I would definitely like to see them one more time...."Tell him to eat a plum....PLUM!" Also Uncle Leo ( "Hellooo"), Jack Klompus (upside-pen-owning semi-nemesis), J. Peterman ("Are you an assassin....or an errand girl sent to collect a grocery bill?)
Newman...Newman...Newman...Jerry's letter-carrying, sausage-making, parking-scofflaw, arch-nemesis. I can't get enough of Newman.
Kenny Banya (sic) ...that's gold Jerry, GOLD. Another nemesis.
Since the main characters have undergone no real changes in the past nine seasons, I see no good reason for that to happen now. No marriages or engagements or moving out of town. I saw a post where someone said they would like to see the series end at Monk's with everyone just discussing the usual stuff...fine by me.
This series needs no closure. Attaching closure to it just because it is ending would seen artificial.
Other nemesis characters who I think deserve a last appearance Lloyd Braun ("Serenity now;...Insanity later"), Braless Sue Ellen Mischke (sic), and for Kramer, his only true nemesis...clowns.