I really think that some kinds of AC shifts need to be scaled with mapsize.
Notably:
Building world units that alter AC on creation
Building World wonders that alter AC on creation
Random events that alter AC
And types that should NOT scale
National or team wonders that alter AC on creation.
Kills with War, or any weapon using <iModifyGlobalCounterOnCombat>
Razing a city
Spreading Ashen Veil
Razing an Ashen Veil City
Sanctifying city ruins
Building a unit at Prophecy of Ragnarok
Hallowing of the Elohim / Elegy of the Sheiam
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The reasons being, that with the possible exception of Prophecy of Ragnarok, all of the things in the latter category, have increased chance to happen, as the map gets bigger. Eg, rituals like Purge the Unfaithful can be done by any nation. The more nations there are, the more they can be used. The 4 things to do with AV and cities, don't need to be scaled because as the map gets bigger, so too does the number of cities. More to burn, more to spread AV to, more ruins to sanctify, etc.
The things in the former category, are one-off events. World wonders and World units can only ever be built once, by one nation. No matter how big the world is, there will never be more than one Sphener or Mardero. So I believe their impact on the AC should be appropriately scaled. Likewise with any world wonders that alter it, such as Pillar of chains. There will never be more than one of it.
Events, I'm not too certain about, but I'd lean towards saying that they should be scaled.