Depending on what you mean by "plausible," that combination sounds like a form of confusion rather than a sustainable philosophical position. If theism is known to be improbable, what could make the theistic hypothesis believable? Is it just the desire for God to exist?

Also, the sheer concept could be plausible but not improbable. The concept's application to reality would be open to an assessment of probability. It's God's existence that we'd be betting on, not the mere concept of God.

We have a concept of Harry Potter, and the story of that magician might be plausible in some respects (assuming Rowling achieved some verisimilitude in the telling of it). But Potter's existence in reality would be improbable. So if there were an agnostic about Harry Potter in your sense, as in someone who thinks the concept or the story is plausible, but who thinks the story's bearing on reality is improbable, I suspect this "agnostic" would just be confused.