Did the creators of Seinfeld ever have an alternate finale in their mind for the show?

Yes

According to Wikipedia:

An alternate ending was also filmed. The jury re-enters the courtroom. When Kramer claims that a woman on the jury is smiling at them, Jerry tells him that she's smiling at them because they might go to prison. When it comes to the verdict, the forewoman of the jury states that the jury finds Jerry, George, Elaine, and Kramer not guilty of criminal indifference which disappoints the testifiers and leaves Judge Vandelay speechless.

This ending can be found as an Easter egg on the "Seinfeld" season 9 DVD set.

The ending has been discussed in some places. From the same Wikipedia page:

Although Larry David has stated he has no regrets about how the show ended, a 2010 Time article noted that the Seinfeld reunion during the seventh season of Curb Your Enthusiasm "was viewed by many as his attempt at a do-over."

This was also referenced by Jerry in the seventh season finale of Curb Your Enthusiasm, saying "We already screwed up one finale" with David responding "we didn't screw up a finale, that was a good finale!"

Having said that, during a Seinfeld roundtable reunion discussion, Larry admitted to understanding the disappointment and said if he were to redo it he would have kept the plot of the finale less of a secret, which only heightened expectations.