One thing’s for certain: Dark Souls II is “a full game,” according to producer Takeshi Miyazoe. Rather than create a game with dangling loose ends that are only tied up by subsequent DLC, FromSoftware chose “to deliver a full game, the full experience, to fans who purchase the package from day one.”
The original Dark Souls was likewise a full game. While its DLC, Artorias of the Abyss, certainly deepened the game’s lore and expanded its universe (namely through the 4th dimension), it is by no means “required” to have a full experience. Hell, most of my friends and I had gone through the game with multiple characters by the time the DLC came out.
Let me put it this way: playing Artorias of the Abyss will probably make your Dark Souls experience richer, and thus better, but the inverse is not true. Not playing Artorias of the Abyss will not make Dark Souls worse. Does that make any sense?
When it comes to DLC for Dark Souls II, the details are vague. Producer Takeshi Miyazoe is quoted as saying:
There is definitely potential, I think. Again, it’s all about fan feedback. Right now we are concentrating on completing what we’ve got, but I think there are next steps on how we can expand on Dark Souls II. So I’m not really ready to say look forward to more information, but I think the potential is definitely there.
So there you have it. FromSoftware will consider making DLC for Dark Souls II if fan feedback is positive (which I’ve no doubt it will be), and only after ironing out any kinks and bugs in the base game.