As fans of "The Originals" know, Klaus (Joseph Morgan) is no longer immortal, after Davina (Danielle Campbell) successful severed the sireline in the previously-aired Season 3 Episode 15. Understandably, Klaus is paranoid that all his enemies will plot his murder and go after the people he loves.
In "An Old Friend Calls," every member of the Mikaelson family is seen fretting about the enemies that are targeting them. Elijah (Daniel Gillies) is even heard saying, "No one is safe," as a bloody Cami (Leah Pipes) looks shocked.
The Mikaelson brother who literally went to Hell and back, is now the only one who is safe. After many episodes of promises and futile attempts, Davina successfully brought her boyfriend back from the dead, and the two of them are seen visiting Klaus.
Although the promo shows Klaus getting shocked at his dead brother's arrival, it is possible that Kol is willing to help his siblings. Despite centuries of bad blood between Kol and Klaus, it looked they had buried their hatchet just before Kol's death. Klaus was in fact heartbroken at his brother's death.
However, due to other pressing matters, none of the Mikaelsons tried to bring Kol back; not even Rebekkah (Claire Holt) who had promised to do so. In fact, for the sake of her other brothers, Rebekkah even stopped Davina from bringing Kol back at one point.
Kol must definitely have some pent-up frustrations due to that, after all, his greatest complaint has always been that he felt like an outsider among Rebekkah, Klaus and Eljah. This time, however, he has someone that loves him, and might grow out of his juvenile behaviour and help his siblings instead of causing trouble for them.