Dragon

Base on what Freya said right before the game ends , she will likely unleash her vengeance onto Kratos and his son .

And the ending also revealed Kratos 's son Atreus as Loki ,  and we knew Loki will play a major part in Ragnarok.

However we still have access to the Tyr's temple post game , which already had all 9 gates showing up. With only Asgard and Vanaheimr locked.
Instead of trilogy it is more likely they will release several expansion and DLC for you to unlock those gates to extend the story to some degree

in the " secret " ending  albeit briefly .  That's right this game actually has 2 " endings " .

The Ragnarok event will likely a full game , in fact the second ending heavily hinted on how it will transpire
According to Mimir , Ragnarok was suppose to take place many many years into the future , the World Serpent was actually travelled back in time and he did not recognize Atreus as Loki but rather commend on how he looks " familiar " .   What Kratos did apparently had a big impact in the apocalyptic event in Asgard and forced Ragnarok to occur 9 months after the endgame.
But before the Ragnarok there will be more stuff to "explain " , such as Freya . She will play an important role in the war no doubt.
Then we have Thor , whom was seen to approach Kratos before the event.
And of course how Atreus will play his part as the biggest bad guy among Norse gods since he is still too young to become Loki

Plus you still have 2 gates to unlock , no reason to put them in the game if you don't get to access them eventually