I disagree with this strict interpretation I've been seeing. The Faerun, Song of Ice and Fire, and Anno Domini mods were each pretty robust if not 100% full conversions of the game. They may have been more generic than specific, but I would absolutely count them as full conversions.
They may be total conversion mods in the sense that they use a completely new set of units, civs, policies, etc., but they have nowhere near the degree of alterations that many of Civ4's finest total conversion mods achieved. Final Frontier not only only had a new set of units, civs, techs, etc., but it also completely changed the city management mechanics, where citizens were allocated to planets (not tiles) and buildings were built on individual planets within a solar system (thus allowing you to hyper-specialize certain solar systems by building certain buildings on all planets way ahead of time). Planetfall completely replaced the terrain system with one based on grades of elevation and rainfall, complete with a replacement of LoS mechanics, river crossings, terrain movement costs, and base map generation to go along with it. Fall from Heaven 2 and its many submods did ridiculous things like having a civ who built improvements instead of buildings (yup, Jotnar existed before Civ6 was even announced), a civ who had a random chance to spawn settlers that were also their way of growing cities instead of using food (Scions), and a civ whose first 3 cities had +1 city radius but all their other ones had a radius of 1 and couldn't build any wonders (Kurio). Both Planetfall and FfH2 had multiple World Congresses with different sets of resolutions, and if a player didn't like following the resolutions of the first congress, they could usually opt to quit and join the other one, or just not partake in either if they wanted to. Afterworld completely replaced the city management system and instead had the player controlling 6 units with all manner of spells and abilities. Civ4:Col, which I'd consider to be a Civ4 mod like how CivBE is a Civ5 mod, competely scrapped the old yield system and instead ran on a system powered by individual goods like the Settlers series.
Compared to what was done with Civ4's mods well before Civ4 ran its course, Civ5's "total conversions" are more like overhaul mods: they're like replacing a car's chassis, seating, and steering system, as opposed to Civ4's total conversions that are like taking out the car's engine and putting it in an entirely new car.
I've posted this in another thread before, but I'll repeat it here: in order for Civ6 to get these sorts of total conversions, they need to release the DLL source code ASAP. The DLL is usually a giant jungle to navigate, so it takes modders at least a couple years to figure out how and where to tweak things, and there is no way to truly replace it. Sure, you can make Faerun mod-style overhaul mods without DLL modding, but if you want modders to come up with stuff that's truly wild, they need to be able to change the game's most core systems. The longer Firaxis delays with releasing DLL source code, the less likely modders will end up producing total conversion mods before general interest wanes in Civ6: just look what happened to Civ5, where the first real overhaul mods (e.g. Anno Domini) only really came into their element the same year Civ6 was announced, and Civ5 released DLL source code after G&K.
