Hypothetical PoC Video Game Design
So, if someone were to hire me to build a Paths of Civilization game, I would probably go with something Paradox-ish. I mean, if I had unlimited budget and access to devs I would probably bring in more of Sid Meier's Civilization elements, but for a game that would actually get made I would go with a more limited design. Within a Paradox style game I would probably do it with a real world map and start centred around the Eastern Mediterranean - the edges would probably be the north shore of the Black Sea, the Balkans, the second Nile cataract, and Bactra - and would run from say 5000 BC - 300 BC. Western Europe and India as well as a timeline extension to say 400 AD could be saved for expansion packs. The map would then be divided up into a large number of provinces, so the look would be very much like if someone decided to make the prequel game to Crusader Kings. Design would also have three distinct phases of early game, mid-game, and late game divided up into Neolithic, Bronze Age, and Iron Age.
The play style would however be very much closer to a combination of the Europa Universalis and Victoria games, only turn based, with each turn being maybe 5 to 10 years. Colonization, infrastructure development, and internal stability would be key goals, but the primary challenge would be balancing long term development with immediate survival. Over developing farmlands can lead to soil degradation and thus agricultural collapse, but you need to keep food production and granary supplies high in order to do things and keep people happy. There would also be a relatively complex resource dependency network, where again you should feel like you always need more resources just to stay ahead of the demands of the priests, bureaucrats, artisans, and warriors that keep your civilization running.
For research I would probably have something like a hybrid of Civilization and Victoria II, with a splash of Master of Orion 2. There would be a number of Main Techs in an interconnecting tree, but each Main Tech would also have a number of Secondary Techs that branch off. While everyone can develop the Main Techs, the secondaries are somewhat random in what actually opens up, weighted by environmental factors. The big thing though would be that tech develops primarily by use. Building lots of farms encourages the growth of farming tech, working with metals grows the smithing techs, and so on and so forth. Being in contact with other civilizations that have techs will also encourage the growth of those techs, dependent upon what policies you and they have. Somewhat related would be social values and policies, which would also develop over time, but new ones can open up with increasing technology, and then you have the question of retaining older but more developed systems or moving to initially less powerful values that have more room to grow.
There would be a fairly robust random and semi-random event system, with a strong emphasis on the environment being ready to kick you in the ribs at the slightest provocation, and your subordinates being total dickheads. In fact, I would put more emphasis on keeping your subordinates in order than necessarily conquering your neighbours, at least until the Iron Age. One of the big things I would emphasize is that there's not enough actions in a turn to do everything, and thus you need to delegate to the AI, which might be less competent than a human player but at least things are happening instead of everything grinding to a halt because the king doesn't have enough hours in a day.
Finally, the combat system would be fairly radically different than most of these sorts of systems. Each province would have a garrison and for the most part wars would be fought by setting garrisons on the edges of your territory to raid neighbours or explore or what have you. The direct movement of units would probably be restricted to a sort of mini-game when engaging other major kingdoms and empires proper, and/or restricted to the Iron Age.
So yeah, if I were to do this, I would start out relatively unambitious in terms of scope, limiting to a time period of the start of agriculture to the early classical period and within the Fertile Crescent. If I had more resources, including developer brain power, I would start by expanding the area a bit more and extending out the timeline to the classical period. More resources still and I would try to add in China (that or just focus on East Asia as its own standalone spinoff). Then if I could I would add in a random map maker, so that there could be more varied gameplay. Finally, if all of the above were working, then I would see about getting somewhat ridiculously ambitions and go for the full Civilization meets Paradox experience and extend out the timeline beyond the classical, hopefully all the way to the Information Age... although the shifts in gameplay along the way might make sensible to split it up into multiple games running off the same engine, each with somewhat different mechanics.
