It's going to be very challenging for the writers to do a finale. The kind of things that would be done in a series finale have already been done during the course of the series!
How about if, in the finale, George gets engaged? No, wait; they did that already.
How about, after years of dating incompatible women, Jerry meets the ideal woman; a woman who is just like him! Then, he can..Oh, damn! They did that once! Oh, well...
Hey! I've got it! How aboout Jerry and Kramer have an argument, Kramer gets offended and moves to Hollywood, and lands a role in a network TV sitcom! This would be a great way to wrap things up! At long last, Kramer finally... UH OH! That's been done TOO!
Maybe George can finally get a steady job. How about something prestigious, like working for a celebrity... OOPS! How COULD I forget?!! He spent a season and a half working for George Steinbrenner!
Of couse, they could have Jerry & George finally sell a pilot to NBC. But they did that too. Of course, the deals always fell through at the last minute. That's what made it funny. If Jerry and George succeed in selling their TV pilot in the final episode, we're all going to expect something to go wrong.
Maybe the last episode can be a parody of every famous series finale ever made! The gang gets sent home from the army, and bid each other tearful goodbyes; after they get home, Kramer finds himself on the run from the law, but finally catches up with the one-armed man; George buys the coffee shop, and then retires and sells it, and then buys it back again; Jerry and George are falsely rumored to be dead, causing Elaine and Kramer to mourn, and Newman to rejoice. After the rumors are squelched, the gang goes on a three-hour tour, get stranded on an island for 15 years, then finally get rescued. After they arrive home, Jerry, George, Kramer and Elaine are tied up by terrorists, with a bomb set to go off in seconds. As a result, Jerry has no time to change, so he is forced to reveal his secret identity to his friends, as he changes into Superman to rescue them.
(If you thought Superman's secret identity was Clark Kent, youre wrong. Clark Kent is a fake secret identity Superman's been using all these years to throw his enemies off track. His REAL secret identity is Jerry Seinfeld. Notice you never see Superman and Seinfeld together?