These are my predictions for SNL's season 39:
What we already know: Bill Hader, Fred Armisen, and Jason Sudeikis are not returning. Nasim Pedrad may also not return; she starred in John Mulaney's sitcom pilot and said publicly that she'd leave SNL if it were picked up. It was passed on by NBC, but that doesn't mean it's dead, and her willingness to leave for that pilot likely (IMO) signals that she'd leave if given an offer for another show.
Vanessa Bayer may be let go before the start of the season. She's my favorite actor on the show, so I hope I'm wrong, and I'll be very mad if it happens. My worry stems from what seemed to be a diminishing of her screen time during the last half of season 38. Of course, a lot of that depends on whether Lorne Michaels thinks someone trying out is funnier, so if the tryouts are duds, it's more likely that we'll see her again.
Kate McKinnon and Cecily Strong will be promoted to repertory status. Kate in particular seems to be considered a rising star, as she gets more screen time than half of the repertory players (with good reason). Aidy Bryant and Tim Robinson will likely stay as featured players. That's if they remain at all- first-year featured players often get pushed out if better people come along. Both of them did very well last season, though, so I doubt anyone will top them.
We already know that Seth Meyers is leaving at Christmas break. With the others leaving, there will be at least four new cast members, and possibly as many as seven. While they're likely to all be featured players, it's possible that at least one will be a rare-case new repertory player, possibly whoever takes over Weekend Update. (My expectation- new Weekend Update person will start featured and will be promoted to repertory starting with the first show of 2014, aka the first show without Seth.)
As for who may be new cast members, I have only one prediction, and that's Colin Jost. He was promoted to head writer for season 38, alongside Seth, who'd done it alone for the two seasons prior. That almost certainly means that Jost will be the new head writer, but I suspect he'll get a promotion to featured player at the same time.
At whom should the folks at SNL look? One name: Beck Bennett. You may not know the name, but you've seen him: he's the deep-voiced guy holding roundtable discussions with children in commercials for Sprint. He knows his way around sketch comedy (take a look at his YouTube channel), and who better to anchor Weekend Update?