Predictions say that Gibbs will be promoted to Director next season ,which will leave his team leader job open for someone new.
What I read into that is the producers are trying to do what they wanted to do with JAG, which was to promote the main stars (who were growing too old for the roles and had enough $$ to not want to work hard anymore) and refresh the show with younger (and cheaper) actors. CBS passed on that, and Fox passed on a similar effort with the X-Files, but neither of those shows was every a blockbuster (JAG was a solid but unspactacular show, peaking at #15 and down to #60 when cancelled).
So, probably:
Vance gets elected Senator, allowing more political themes (and more political preaching, which would please CBS).
Gibbs goes over to the director role, allowing him to continue to appear (and perhaps appear in the two spin-offs which could use the boost) without working nearly as hard as he has more money than he can ever spend.
Probably Quinn gets the team leader job, note that the character was specifically written as older and more experienced that McGee, which can lead to a resentment and anst theme relative to him.
Probably Mallard is aged out, replaced with Palmer.
The Reeves character leads to more spy based themes, McGee returns to his computer geek role and the Torres character takes over from Gibbs as the male model action figure.