The last fans saw of Nessa and Smithy, played by James Corden, was in the 2019 Christmas special, where Nessa went down on one knee, asking Smithy to marry her.
The trailer aired on Saturday night did not make it clear whether the couple has since tied the knot.
But one of the show's biggest mysteries - what happened during Uncle Bryn and Jason's fishing trip - may finally be revealed in the show's Christmas special.
We heard one character, thought to be Gwen (Stacey's mother) saying: "Thirty years ago you two went on a fishing trip that almost tore this family apart," suggesting fans may find out why.
All the recurring characters were seen in the clips, including Gavin and Stacey (played by Mathew Horne and Joanna Page), who got married in the first season of the show in 2007.
The trailer ends with the characters dancing and knocking back a round of shots.
Fans took to social media after the trailer dropped, with one X user writing: "This looks so so good."
"Strictly, you're killing me here!," wrote another.
"I can't wait to watch it but I'm not ready for it to be completely over," a third commented.
During her cameo as Nessa, Jones explained the terms and conditions of voting for Strictly's winners, and told host Claudia Winkleman: "I see you're copying my hairstyle still, tidy."
She added: "Please don't try and vote if you're watching on demand, you'll just make a fool of yourself."
The hit TV series, co-created by Corden and Jones, aired between 2007 and 2010, and returned in 2019 with a one-off festive special