If next year is the last- How about going out with a satire all season of how most shows go out- completely losing focus and making all the characters act in ways that are completely wrong and getting into completely unrealistic situations...
Wait a minute- never mind...
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Seriously, how about having all the characters do all the things most sitcom characters do: Elaine and George fall in love, George gets Elaine pregnant, she has a baby, etc. (and they could have Elaine just show up pregnant in the season opener with George as the father) Jerry shows up in the season opener married. Kramer has a real job and has become really responsible. They could probably sustain that joke for a year.