Amazing how quickly the board has moved on from Ryan Callahan.
Cally is one of my favorite Rangers of all time. That said, the OP asked who the core is if the team rebuilds. Callahan's style of play has been taking a toll on his body for years. He's going to be more expensive than his production (due to missed games that will happen every year), and even with his other attributes, leadership and intangibles are difficult to express from the IR. If this team IS going to re-build, then keeping Cally makes no sense. He'll be a complete shadow of his former self by the time the rebuilt team would be ready to compete, and he has enough value now to improve the odds on the success of a rebuild.
IF the team decided to blow it up and rebuild:
I definitely keep: Lundqvist, McD, MDZ, Moore, Kreider, and Stepan. These six guys are young enough and talented enough that they will be a good veteran core for the rebuilt team (all--with the possible exception of Lundqvist--would be in their primes when the influx of youth will be ready).
I definitely trade: Girardi, Staal, Stralman, Callahan, Nash, Hagelin (I think people assume Hags is younger than he is) and Brassard. These seven guys have two things in common. They all have great trade value, and they will all be on the downswing of their careers before a rebuilt team would be ready. These are complimentary players for a
current contender. Not a future one. Spin them off for 1sts (especially in 2015) and high end prospects.
I keep prospects like Kristo, Miller, McI, Fasth, Lindberg--basically guys that might have NHL careers.
I trade guys like Boyle, MZA, D. Moore, Pou, etc if the price is good. They don't hurt the team at all, but they also won't bring back the same kind of high end return that you'll get from the "definitely trade" list.
Finally, I fire AV yesterday. He was brought in to take a good team to legit contender status. If we go full rebuild, we need a coach with more of a rep for developing young players who is brought in knowing that that is his job.