I think I will give it a try. I modified the entries in the civ5worlds.xml file in the Expansion2 ... GameInfo folder:
<Row>
<Type>WORLDSIZE_HUGE</Type>
<Description>TXT_KEY_WORLD_HUGE</Description>
<Help>TXT_KEY_WORLD_HUGE_HELP</Help>
<DefaultPlayers>
18</DefaultPlayers>
was 12
<DefaultMinorCivs>
36</DefaultMinorCivs>
was 24
<GridWidth>
160</GridWidth>
was 128
<GridHeight>
100</GridHeight>
was 80
<MaxActiveReligions>
9</MaxActiveReligions>
was 7
<FogTilesPerBarbarianCamp>35</FogTilesPerBarbarianCamp>
<NumNaturalWonders>9</NumNaturalWonders>
was 7
<UnitNameModifier>0</UnitNameModifier>
<TargetNumCities>
10</TargetNumCities>
was 6
<NumFreeBuildingResources>7</NumFreeBuildingResources>
<BuildingClassPrereqModifier>100</BuildingClassPrereqModifier>
<MaxConscriptModifier>75</MaxConscriptModifier>
<TerrainGrainChange>1</TerrainGrainChange>
<FeatureGrainChange>1</FeatureGrainChange>
<ResearchPercent>130</ResearchPercent>
<NumCitiesUnhappinessPercent>
40</NumCitiesUnhappinessPercent>
was 60
<NumCitiesPolicyCostMod>
4</NumCitiesPolicyCostMod>
was ? - I shoud have mad a backup first
<NumCitiesTechCostMod>
2</NumCitiesTechCostMod>
was ?
<AdvancedStartPointsMod>120</AdvancedStartPointsMod>
<EstimatedNumCities>200</EstimatedNumCities>
<IconAtlas>WORLDSIZE_ATLAS</IconAtlas>
<PortraitIndex>5</PortraitIndex>
</Row>
I didn't add 24 civs, because already 18 will probably increase the length of calculating between turns.
The number of beliefs in VP allows for only 9 religions.
I don't know what TargetNumCities does, but I increased it, as it was gradually increased in the settings from tiny to the original huge.
I reduced the NumCities... entries, because I read somewhere it could become difficult otherwise to have the big new world settled with cities.
Using Really Advanced Setup (v 15) and Vox Populi (3.5.2), the world builds up without problems, and InGame Editor+ (v 45) shows the intended number of Civs and City States is present.
I will start to play next week (a King's Marathon).
Is there any advice you can give on whether or how to alter/improve these settings to avoid dissapointments during my game ?