https://deadline.com/2019/04/the-big-bang-theory-finale-hints-character-endings-contenders-emmys-1202590808/

The final Big Bang episode, airing May 16, "has not been written completely yet; it’s been outlined," says director Mark Cedrowski. He promised that the series finale won't feature anything drastic. “It’s going to be the last episode and it’s going to end, but people’s lives still go on,” he says. “The characters will be giving the idea that their lives continue on. It’s not going to be an atomic bomb where things blow up and you never see anybody again.”

Makes sense. It also leaves room to come back!

Obviously none of us can say anything for certain, but it sounds like things are headed in the right direction. I think one of the things fans enjoy most about the series is the sense of community that exists around the core 7 characters. That community is the heart and soul of the series, and I think that is what moved the series to a higher plain, one reserved for a select few series over television's long history. The image of the 7 of them sitting around Penny and Leonard's apartment eating and talking about their lives should be what endures (although I wouldn't mind seeing that elevator get fixed as suggested above).

From the article, it seems they're still determined to leave Penny's last name a mystery. One thing they could do is say Penny is actually her (and Wyatt, etc.) last name, so that the mystery is really about her first name.

That will be a real letdown. And a suggestion like that would be stupid. It'll just be sending a message that it's okay to leave storylines unresolved forever when it clearly shouldn't be.