Thanks David. Yes it is a challenge. And the "right" fit has not jumped out yet.

I had some background in some of these things you mentioned because of Graham Hancock's books 15-20 years ago starting curiosity. As an engineer I often look at things and think how did they do that?

Last summer myself and my two sons (13, 9) took the Salisbury Cathedral Guided tour. The best tour I have ever taken. The boys lover it. Just a small group (10) with a knowledgeable guide. We were taken behind the scenes, all the way up to the spire. You see 1000 year old beams. The oldest working clock mechanism in the country. Mediaeval craftsmen in the raw. The fixes to structural problems as the building grew and settled. A real delight. (If any of you ever get the chance, take the tour.)

It gets the imagination going in similar themes you mention. Finding a setting that is viable.

Well natural things are allowed. Guardianship of a crystal cave that displays some rare isotopes giving correlation? Does not have to be organic.

For reasons of plot I know the preference is not the middle east, but the far east. The migrations of peoples from Central Asia over the last 2000 years. There were various waves, not just the more famous Tartars, and Mongols at the gates of Vienna.

But nomadic horse driven life presents a problem, not least that they did not leave much behind. Not even writing.

There is a problem with pyramids. They are losing credibility. There is the Bosnian one currently, it just attracts too many people not fully grounded in scientific/engineering archaeology. The mysticism attached to them is like an infection. Best to keep away.

So yes the struggle is ongoing. However, even just this interaction helps focus the mind. I am great believer is positive tension creating results.