My source is Entertainment Weekly, a publication I don’t normally read. But there was a big special souvenir edition for the last Seinfeld episode, with lots of behind-the-scenes information. I think that Jeremy Piven was up for the role of George. One of the would-be Kramers was in the two-part episode about “Jerry”, the TV show that is developed by Jerry. You have to remember that Rosie O’Donnell looked a lot different 25 years ago and had a different image. I think she was basically a stand-up comedienne, wasn’t speaking out on politics or anything.
Googling it, Patricia Heaton and Megan Mullally were also considered for the role.
I like Jeremy Piven but it's hard to imagine anyone doing better than Jason Alexander in that role. Piven played the actor playing George in "Jerry".