Olive Branch Books has no plans to add another level to Telling God’s Story (TGS). In the elementary years, the student is interested in hearing the stories from the Bible. The young student has questions about what happened, much more than why something happened.
In the middle school years, however, this changes. Students start making connections between stories, and striving toward deeper understanding and personal conviction. At this stage, reading the Bible directly, perhaps with a commentary suited to your family's beliefs, is the way to go. Perhaps you are affiliated (if not by attendance, by belief) with a particular denomination or belief system. If so, look for the denomination's or spiritual group’s education materials, especially those targeted at the pre-teen and young teen.
Take a look at Peter Enns' website. He is the author of TGS, and you might find continuity among his works.
Susan Wise Bauer’s article “A Neutral Education” is interesting and helpful in this regard.
You could also post your question, stating your specific requirements, on the Well-Trained Mind Community Forums, probably in the K-8 or the General Education Forums. You will get some good suggestions (and probably some discussion!) about the merits of various curriculum and offerings that are available and what might best suit your family.
When you make your post, be as specific as you can. Keep the focus the discussion by saying something like this: "I'm not asking for a religious debate; I'm looking for curriculum that rests within these parameters."