I don't think there is a "core" during the rebuild. Anyone is available for the right price according to the reports, so why identify a core to build around? Sure, a guy like Kreider isn't exactly easy to replace, but few players are truly "easy" to replace. Kreids will be 27 to end the season. What does he look like in two years when the Rangers are ready to start competing again? He's a group III two years from now, so that needs to be considered. I think he's a guy who will probably play well into his 30's in some capacity, but you have to look at his value objectively over the duration of his contract.
Fast is a guy who I love and he fills a great role on the team. He's on a great deal considering what he brings to the table, but again, he's a UFA after the season in which the Rangers will likely start really competing.
So the team needs to step back and look at situations like that. Do they have more value as pieces who will be with the team through the rebuild, but will need new contracts right as they start to maybe make a playoff run? Or are they going to give the Rangers the most value in trades that get them cost-controlled pieces they can build around for the next 5 or even 10 years?
Step 1: Evaluate the market, and the pieces you currently have. Step 2: Determine what kind of team you want to build and how those pieces fit in. Step 3: Start building.