I have been playing civilization since Civ 1 in the 1990's and spent particularly lots of time on Civ 4, which in my opinion is the best of the series (haven't played Civ 6 yet). I have an academic background in history and archeology, so I hope I can suggest a few helpful tips on how to make the game even more realistic without losing in fun. Note that I always play with real maps (mostly huge world map).
1) Start the game at 10,000 BCE instead of 4000 BCE
In all version, the game starts in 4000 BCE, which corresponds to the Late Neolithic period in Europe. This is when agriculture finally reached Britain and Scandinavia, 2500 years after it starting spreading to Southeast Europe. The Copper Age started around 5800 BCE in Southeast Europe and the Middle East and had spread to a large part of Europe by 4000 BCE. In East Asia and India, farming was also widespread by then. The Bronze Age started soon afterwards, around 3500 BCE (a few turns after starting the game).
It doesn't make much sense to start in 4000 BCE if we are going to research such technologies as agriculture, pottery, the wheel, animal husbandry, the calendar, mining, and so on. All these technologies would have been long acquired by 4000 BCE in places where ancient Eurasian civilizations arose.
2) Drop Paleolithic technologies
One of the things that doesn't make any sense is that we have to discover hunting, archery and sailing. Humans have been hunting since the time of Homo erectus one million years ago, long before Homo sapiens and Neanderthal even came into existence. Check this list of prehistoric inventions. Spears were invented 500,000 years ago and bows & arrows at least 65,000 years ago, before modern humans colonized the Middle East and Europe. Humans very reached Australia via Indonesia some 40,000 years ago and they were using boats to do so. So it is sure that sailing had been invented by then.
3) Adapt movement speed to the age and restrict movement in the wild
One thing that has long bothered me is that scouts or other units can explore territories extremely remote from their own civilization, or any civilization for the matter, as soon as the game starts. In a few turns Europeans can meet East Asians, before horse riding or the wheel were even invented. Explorers can stay in the middle of Siberia or in the desert for as long as they want with no food or connection to their country for centuries and even millennia. That doesn't make any sense. On the other hand, later in the game, units take years to cross Europe, even with horse-riding units, when in reality it would have taken a few weeks, so much less than a single turn in the game. We need to find a better balance for movements.
What I propose is that the movement range of each unit vary according to their historical era. In ancient times people didn't move much and would not have sought contact with distant cultures/civilizations at the other end of the world. Settlers especially would have spread very slowly. I would force all units to move only 1 square per turn, and require two turns to cross rivers and hills. They wouldn't be able to enter deserts and jungle at all except along a river on the square (like the Nile for the Egyptians). From classical times the movement would be similar to the current default, but with slower settlers (to avoid Greeks or Persians colonizing Siberia or other nonsense) and faster horse-riding units (at least two or three times the default range, as this is more in line with the real difference between a foot soldier and a horse rider). From industrial times, units would be twice faster, and from modern times even faster.
I don't think that roads are very important to increase the speed of horse riders. Mongols rode fast across the Eurasian Steppe without roads. Therefore I would remove the movement bonus for roads. Instead road connections would be required to trade with other civilizations, unless they have a maritime connection through a harbor. Trade would be possible between two civs if an intermediary civ with open borders has road connections linking the two. This way Europeans won't be able to trade with Amerindians until both have harbors, or with East Asians until all Central Asia has been colonized and a road network is established from China to Europe via Central Asia and the Middle East (for example, with the Silk Road in mind). Roads would still be required to connect resources and would provide higher income on any square that has one.
Additionally, units that leave the boundaries of their civilization or another civilization with open borders would lose HP at each turn and die within three turns if they aren't back to a country that can provide them with food. This way explorers won't be able to cross Siberia until at least some relay cities are founded there or they have modern vehicles that allow them to cross in one turn.
I am also in favor of explored squares of maps disappearing after a few turn of exploration until the development of cartography, as in Realism Invictus. It just doesn't make any sense that wild lands explored by some long dead people hundreds of years ago remain known if there is no way to record it.
The aim of these measures is to prevent contact between very remote civilizations on different continents for as long as possible, especially before Classical or Medieval times.
1) Start the game at 10,000 BCE instead of 4000 BCE
In all version, the game starts in 4000 BCE, which corresponds to the Late Neolithic period in Europe. This is when agriculture finally reached Britain and Scandinavia, 2500 years after it starting spreading to Southeast Europe. The Copper Age started around 5800 BCE in Southeast Europe and the Middle East and had spread to a large part of Europe by 4000 BCE. In East Asia and India, farming was also widespread by then. The Bronze Age started soon afterwards, around 3500 BCE (a few turns after starting the game).
It doesn't make much sense to start in 4000 BCE if we are going to research such technologies as agriculture, pottery, the wheel, animal husbandry, the calendar, mining, and so on. All these technologies would have been long acquired by 4000 BCE in places where ancient Eurasian civilizations arose.
2) Drop Paleolithic technologies
One of the things that doesn't make any sense is that we have to discover hunting, archery and sailing. Humans have been hunting since the time of Homo erectus one million years ago, long before Homo sapiens and Neanderthal even came into existence. Check this list of prehistoric inventions. Spears were invented 500,000 years ago and bows & arrows at least 65,000 years ago, before modern humans colonized the Middle East and Europe. Humans very reached Australia via Indonesia some 40,000 years ago and they were using boats to do so. So it is sure that sailing had been invented by then.
3) Adapt movement speed to the age and restrict movement in the wild
One thing that has long bothered me is that scouts or other units can explore territories extremely remote from their own civilization, or any civilization for the matter, as soon as the game starts. In a few turns Europeans can meet East Asians, before horse riding or the wheel were even invented. Explorers can stay in the middle of Siberia or in the desert for as long as they want with no food or connection to their country for centuries and even millennia. That doesn't make any sense. On the other hand, later in the game, units take years to cross Europe, even with horse-riding units, when in reality it would have taken a few weeks, so much less than a single turn in the game. We need to find a better balance for movements.
What I propose is that the movement range of each unit vary according to their historical era. In ancient times people didn't move much and would not have sought contact with distant cultures/civilizations at the other end of the world. Settlers especially would have spread very slowly. I would force all units to move only 1 square per turn, and require two turns to cross rivers and hills. They wouldn't be able to enter deserts and jungle at all except along a river on the square (like the Nile for the Egyptians). From classical times the movement would be similar to the current default, but with slower settlers (to avoid Greeks or Persians colonizing Siberia or other nonsense) and faster horse-riding units (at least two or three times the default range, as this is more in line with the real difference between a foot soldier and a horse rider). From industrial times, units would be twice faster, and from modern times even faster.
I don't think that roads are very important to increase the speed of horse riders. Mongols rode fast across the Eurasian Steppe without roads. Therefore I would remove the movement bonus for roads. Instead road connections would be required to trade with other civilizations, unless they have a maritime connection through a harbor. Trade would be possible between two civs if an intermediary civ with open borders has road connections linking the two. This way Europeans won't be able to trade with Amerindians until both have harbors, or with East Asians until all Central Asia has been colonized and a road network is established from China to Europe via Central Asia and the Middle East (for example, with the Silk Road in mind). Roads would still be required to connect resources and would provide higher income on any square that has one.
Additionally, units that leave the boundaries of their civilization or another civilization with open borders would lose HP at each turn and die within three turns if they aren't back to a country that can provide them with food. This way explorers won't be able to cross Siberia until at least some relay cities are founded there or they have modern vehicles that allow them to cross in one turn.
I am also in favor of explored squares of maps disappearing after a few turn of exploration until the development of cartography, as in Realism Invictus. It just doesn't make any sense that wild lands explored by some long dead people hundreds of years ago remain known if there is no way to record it.
The aim of these measures is to prevent contact between very remote civilizations on different continents for as long as possible, especially before Classical or Medieval times.