More content would be really nice, but I feel that the locked realms and anything left open in the story point more toward a sequel than DLC. If we explore all the 9 realms right now, that would be a pretty big game, and there wouldn't be as much left for a sequel or sequels. I know they hinted at other pantheons, but I don't want to jump away from the Norse setting too quickly. The game is already longer than any previous GoW game. Based on interviews with Cory Barlog, he's told the story he wanted to tell for this game. Additional encounters added in DLC wouldn't fit into that. A traditional expansion might be appropriate but that isn't really a thing anymore.
Muspelheim can be reached fairly early on, and Niflheim not too close to the end. You don't have to grind them at the end with no purpose after completing everything else, even though most people save them for the end and they are much easier at that point. If you do save them for the end, they aren't too different from the challenges in previous games, which required you to complete the game once to unlock and didn't offer any rewards.