I'm with you on missing John and Marlena. And, there are so many places where they would fit right into or at least enhance the story lines.
John could definitely relate to what Melanie is going through. He could tell her lots of stories about not knowing "who you really are." And about facing the reality of finding out one day that the family you thought was yours is not your family after all.
They wouldn't have to be the central story line. After all, the DiMera "vendetta" is pretty much over. John is now Stephano's half-brother so he could almost serve as a "conscious" to older, sickly Stephano who seems to just want to be a Grandpa now.
Brady definitely doesn't belong in the Kiriakis family. He should be working for his Dad and being groomed to take over Black Enterprises, maybe even learning to fly a jet.
Seems like what little bit of time John spent with Nicole, he always had a way of guiding her in the right direction. He never judged her and understood her pain better than any of the other characters.
He always had a way of "being there" for Sami whenever she was in the most trouble. I can't believe he didn't show up for the Sydney kidnapping fiasco.
And Marlena..geez! There are so many people who need Marlena's psychiatric expertise to help them sort their feelings out. Stephanie, Melanie, Sami, Rafe, Nicole, Brady, EJ, Bo, Carly, Hope..even Vivian and Kate! And, Maggie sure could use a close friend right now.
As for the teenagers...I think it would be better to unite them right now. Have them all working together for a common cause of something...What they are doing with them now just isn't working.