I believe there are already ruins model in-game, in Red Death scenario. Is there a mod that spawns them after every city raze? Seems like an easy script to do but I've never modded CIv before and not sure where to find the reference material for scripting (have no idea how it works).

I guess it should look somewhat like this?

function OnCityRazed(locationX, locationY)
local plot = Map.GetPlot(locationX,locationY);
SetImprovementType(plot, CityRuins);
end