I thought I had watched all of NCIS since the beginning. So, why were some characters introduced and then dropped? (I know that they lost a showrunner, but whoever followed in that niche must have watched a few previous years.)
Vance's son: could be an age now where he's really interesting.
Ellie's family, especially the brothers, lots of effort went into her background.
Gibbs' hometown. It's inspired a few episodes, but there's more there that is possible, just because it's not that far from D.C.
Emily. Don't leave her hanging in a drug influenced life, Uncle Gibbs.
Kasie - needs to begin to fit into the analytical community in continuing education.
Abby's brothers- are out there somewhere.
And some of Ziva's contacts.
And on and on.