Once you've scattered Fey's ashes in the mountains of Jotunheim, you'll get a mini-credits sequence. Think you're done? Think again Kratos. Instead, you'll need to go home and take a (long) nap. Go to the bed and interact with it to send Kratos and Atreus to sleep for actual "years". They've been through a lot, so they really deserve it. What happens next though, is the real God of War ending.
SPOILERS
It's not a long cutscene but it is incredibly significant. Thunder and lightning rages overhead, waking Kratos and Atreus from their slumber and turning the sky a strange shade of blue. Kratos flings open the door, and outside in the clearing stands a hooded figure. Kratos screams against the storm, "Who are you?"
The cloak flutters back to reveal a very important hammer. Thor, anyone?
Kratos and Atreus have presumably slept through the whole of Fimbulwinter and awake sometime on the cusp of Ragnarok. It's unclear whether Thor has arrived to seek revenge for the murder of his two sons, Magni and Modi, or to get help from Kratos and Atreus for the impending Ragnarok. Only time, a sequel or some mammoth story DLC will tell.