You could start by researching egyption architecture. That way you can more accurately represent egyption ruins (IE people are less likely to question what you have produced for them as they'll reference what they have previously learnt about egypt and compare it to your map in order to suspend their belief of the environment. Hopefully finding an accurate representation).
Secondly, don't do a cp_egypt and leave spytech out of the equation. Integrate some hidden spytech bases inside some of your buildings to keep within the TF2 narrative.
Asking what to include really misses the point of detailing, choose a theme, and that theme generally dictates what is included in your map. Excavation of a tomb? an ancient town? an ancient irrigation system? It's usually nice to include wood with your ruins to show a recent inhabitance and acitivity of the local area. Plus it contrasts with all the sand and stone (dirty yellow).