Marlena and John's Daughter (SPOILERS)
In the new novel "A Secret in Salem", John and Marlena learn they have a grown daughter named Charley Gaines. The man who raised Charley was friends with Stefano DiMera. Mr. Gaines' wife desperately wanted a daughter but couldn't conceive, so as a thank you for investing his money well, Stefano used specimens harvested from Marlena and John when they were held by Stefano to create this girl named Charley. Charley was carried by Mrs. Gaines and upon her death, Charley discovered she is John and Marlena's daughter.
These books are considered canonical, so hopefully Days of Our Lives casts this character soon and introduces her to the series.