The lacking of a useful 4th line or role players throughout the line-up really hurt the team in the 'playoffs'.
@Mikos87 nailed it above. They need to find, probably, 2 veteran players who relish that role of playing a hard-nosed game and shutting down the other teams best players. I've mentioned Brian Boyle before but it's that type of guy I'm talking about.
With that is going to come with pushing a player or two both up in the line-up or out of the line-up. The Rangers and Gorton have done an admirable job of adding quality young players like Gauthier, Howden, Lemieux, Kakko, Chytil and even Kravtsov. the issue with having those guys be in your bottom-6 is that practically none of them can be relied on for shut-down minutes. One or two of these guys will need to push their way up the line-up or there will need to be moves made.