Well, he's definitely not abstract, since in essence he is the sum of all the realms. The realm and races are comprised of both physical and spiritual entities, so he would be a material-spiritual duality. Much like a human being, really.

But remember, spiritual and abstract entities are not the same thing. Contrary to what a lot of people think, spiritual entities are still categorized as substances.

I think that with the way they've been progressing in the story, I would imagine that delving more into the Elder Gods and the One Being is the next logical step to take. So far, we've only gotten little details here and there, so hopefully, we'll get more of an explanation in later games.

I hope so too. I want to see how The One being looks like too.