Suggestions, going through all the regional, temporally, and size/effort appropriate megalithic sites:
Ishtar Gate
Catacombs of Kom el Shoqafa
Obelisk of Axum (Though the actual one is 4 century AD they've were building them since at least the 7th BC)
Temple of Ramesses
Naqsh-e Rustam (also includes the Naqsh-e Rajab)
Temple of Seti I
El Deir at Petra
Hypogeum of Ħal-Saflieni (its in Malta, where there are many
Megalith Temples)
Baalbek
Dol-de-Bretagne Menhir
Luxor_Temple
Fortress of Mycenae
Göbekli_Tepe
Fortress of Sargon
Nergal Gate (most ornamental gate in Ninevah)
Sebastos Harbor
Didymaion
Nemrud Dagi
Yazılıkaya
The Long Walls
Thinking up some possible attributes for them...
Ideas:
Yazılıkaya - Hittite temple where the gods of other cultures are shown in procession:
-High Culture boost, no resistance in captured cities.
The Long Walls - famous Athenian defenses.
- 100% defense boost to this city, +100% trade route income, needs harbour
Sebastos Harbor - largest man-made harbour, built by Herod in Israel.
- Moai Statues ability? or some sort of trade or ship production boost
Nemrud Dagi - complex with huge statues of various gods built in Anatolia after the Selecuid defeat
- Sistine Chapel ability?
The Didymaion - built in Ionia to house the oracle of Apollo
- Apollo was the god of colonies and disease, so 1/2 extra health in this city + Forbidden Palace effect
Nergal Gate - representative of Ninevah
- High culture and some sort of military boost, perhaps giving all new units in this city Drill I?
Hypogeum of Ħal-Saflieni - incredably ancient Maltese temple
- One of the first wonders in the game, small boost of +2/3 happiness and culture to give an early leg up to whoever builds it.
Luxor_Temple - fancy pants egyptian temple complex
- Notre Dame ability
Fortress of Sargon - powerful and artistic defense for a city
- Chichen Itza ability
Göbekli Tepe - even older neolithic holy site with animal carvings
- If can be coded, have you units always win vs animals, otherwise the inversion of Hal-Saflieni, providing a few health and culture for an early boost.
Dol-de-Bretagne Menhir - Archetypical Celtic standing stone
- Happiness or culture boost to monuments, or give it the current RFC pyramid ability and give the Pyramids something more impressive.
Fortress of Mycenae - could be renamed the Lion Gate, emblem of early greeks
- Ability bonus to units built here, Shock is probably a good one to show the Greeks infantry power.
Baalbek - a temple first to Baal, then greeked to Helios and then romaned to Jupiter, oracle consulted before going on wars.
- Cities with state religion have less war weariness.
Petra Complex - rich religious site on major trade routes
- 50% Gold/free merchants and several merchant slots
Temple of Seti - built to commerorate the victory at Kadesh and celebrates Seti's tactical awesomeness
- speeds Great General occurence, units built here get free sentry promotion.
Temple of Ramesses
- Boosts all Artists in a way similar to Ankor Wat does for Priests (+1 sci, +1 culture)
Obelisk of Axum huge markers for underground burial complexes
- 100% culture ability (the national gallery now as a world wonder)
Naqsh-e Rustam Persian temple site from the Achaemenid period.
- High Culture, extra stability on city conquest
Catacombs_of_Kom_el_Shoqafa - ancient burial site in Alexandria, possible to rename to the Hall of Caracalla
- reduces war weariness
Ishtar Gate
- High Culture, gives a cultural bonus to all walls, health bonus to its city.