They've focused a lot on the characters' parents and family a lot the past few seasons.
Season 11 introduced Bubble Bass' mother as a recurring character. While Bubble Bass isn't an official main character, he might as well be one and his mother has her fair share of appearances.
Sandy's Nutty Nieces introduces Sandy's sister and her nieces. Saving Bikini Bottom: The Sandy Cheeks Movie then introduces her mother, her father, her grandmother, her brother Randy, and her younger twin brother and sister. They will appear again in Sandy's Country Christmas.
Patrick's parents, Herb and Margie, had one final appearance in Patrick's Tantrum before getting a redesign and new names as Cecil and Bunny Star. They also act as main characters in The Patrick Star Show. Patrick's Grandpa was also introduced and Squidina replaced Sam as his sister.
Margaret and Harold SquarePants have many more major roles such as Ma and Pa's Big Hurrah and Sheldon SquarePants after barely appearing for 5 Seasons. Grandma SquarePants even returned for a single episode after her actress retired.
Squidward's Grandma is a supporting character in The Patrick Star Show, and his mother got a redesign in Momageddon (an episode all about the characters' moms), and his father made his series debut in the same episode. And Jeff Tentacles will get his own major role in an upcoming episode.
Mr. Krabs' mother returned in Momageddon and his father appears in both Momageddon and Senior Discount after being presumed dead earlier. Senior Discount also reveals many old and ancient relatives of Mr. Krabs and Old Man Jenkins.
Plankton's mother and father appeared for the first time in their current forms in Blood is Thicker Than Grease and appeared again just a few episodes later in In the Mood to Feud. Many of Plankton's cousins also returned again after a long hiatus.
Narlene and Nobby's family appears in In the Mood to Feud as well.
Karen had a baby, Chip, in Karen's Baby. He hasn't appeared since though.
Mary's Mother from Chocolate with Nuts became a recurring character after a 9 Season hiatus.
Nosferatu got a son, Kidferatu, in Kamp Koral: SpongeBob's Under Years.
Concept art for Sponge on the Run shows at one point they considered showing Gary's parents.
They've really been filling out the characters' family trees the past few seasons and making many of them a bigger part of the series.
