You could just create a modmod in which those civilizations are deleted entirely from the game files, with no other changes, right? It would be a bit different/more complicated than a scenario maybe, but straightforward.
Hmmm.
Let's see, I don't know if you'd need any NEW civilizations. The Muslims who spread from North Africa to colonize the emptied-out land of Europe could be adequately handled as a separate Berber civ, I think; that'd be the most important thing to make sure to ADD rather than subtract.
...
Civilizations you'd need to remove on account of not existing due to the plague that wiped out Europe:
-Americans
-Brazilians (arguably, since they present as Europeans in later eras)
-Celts
-Dutch
-English
-French
-Germans
-Greeks
-Hungarians
-Polish
-Portuguese
-Russians
-Spanish
Civilizations you'd need to remove on account of being dead in the water or effectively dead at game start:
-Amorites
-Assyrians
-Byzantines
-Egyptians (uncertain; Egypt was effectively part of the greater Arab world by this time)
-Hittites
-Kushan
-Phoenicians
-Romans
-Sumerians
...
Note that I think you SHOULD include the Hebrews as an optional 'revolt only' civ, because there would still be plenty of Jews in Rice and Salt, living all over the Middle East.
Hm. Make sure to dot ruins tiles and a few scattered surviving improvements across Europe. Incentivizes colonization and makes it a bigger starting advantage.
The Islamic world needs to be split up among:
1) Berbers in much of North Africa, these are the people who in the canon Rice and Salt timeline play the largest role in colonizing Europe.
2) Turks in Anatolia and possibly the Balkans(I'm a bit unclear about what happened to the European provinces Turkey had already captured by the point of departure that takes place around the death of Tamerlane).
3) If you can have multiple Mongol civilizations, having Central Asian Mongols and Timurid Mongols occupying much of what is now the Middle East would be interesting. If not, maybe you
4) Historically at game start India was largely independent; the dynasty that later became the Mughals muscled in on them later, but that was basically just replacing one set of Muslim overlords in northern India with another. "India" occupying northern India and Dravidians occupying the southern part of the subcontinent sounds pretty good. India needs to be very economically productive to reflect its extreme historical importance in the Rice and Salt timeline- it should be, e.g., rich in productivity-boosting resources.
5) You'll want African and 'East Asian' civs in all the obvious places, and several Native American civs who have North America all to themselves. The challenge of a North America game start is catching up with the Eurasian powers technologically. Maybe you can have events that gift them free Scientist specialists or something?
6) In particular the Iroquois, Chinese, Aztecs, and Japanese are specifically plot relevant to the canon timeline, but you need more than that to fill out the globe adequately. Africa was... largely ignored by the Rice and Salt timeline as far as I could tell, which I feel is kind of a downer and can be fixed here because History Rewritten makes jungles economically productive and has a wealth of African civs whose unique units are highly competitive and relevant at game start.
Sheer number of civilizations is a potential obstacle; you want things to be interesting and not leave anyone a whole uncontested continent, but you also want
