One TEENSY WEENSY little problem. Writing is a very important tech and even if you are on a continents map, you could be an alone major civ on a continent seperated by ocean. Without writing, no libraries. Without libraries, astronomy will take ages. By the time you meet another civ, they'll be in the modern era and your game will be stuffed. How about extending the C to civ OR city state. If you don't have either of those then that's just rotten luck and the game won't be worth it.
This issue can be partly addressed by adding tech diffusion, which either allows you to a) research any given tech faster depending on how many other civs with whom you have open borders also have that tech, or b) increases the chances that you will spontaneously acquire that tech each turn depending on the same factors.
However that may not fully resolve the issue, and imo you've touched on a larger issue which needs to be addressed in order for conditional technological advancement to work: the whole emphasis of the game needs to be shifted so that the focus is primarily on
standing the test of time. It should be much more inherently challenging for a civ to
survive in the long term, and the player should be rewarded (through score and in-game rewards) according to how long they are able to keep their civ "in the game" so to speak. This way a game can still be worth it even if you've had an isolated start and are therefore less advanced than rival civs: you are rewarded for surviving as long as you did (even if you're game ends with you getting conquered by a rival civ), and at the same time it is more difficult for all civs to survive and prosper long enough to become highly advanced and dominate the world.
It could be argued that the industrial revolution and technological/scientific explosion of the last few centuries IRL were the result of a string of lucky historical accidents. It would be cool imo to sometimes see games where no civ ever survives long enough or becomes prosperous enough for this to happen even over the course of several millenia. There needs to be less emphasis - both on the part of game designers/modders and players - on using technological sophistication as a measure of how 'great' a civ is. Civilization is ultimately a huge collective problem-solving excercise, and the object of the game should be for leaders of civs to solve the various problems (drought, floods, resource depletion, wars, plagues, economic problems, political instability etc) which civs have to face. Technological advance should be one of a number potential strategies/options available to civs to solve any problems which might arise in the course of a game.
Therefore, there should just be MAJOR detriments, not complete cutoffs, for not having horses for horseback riding- for we would have ridden stuff, just not horses. And if you see another civ with the tech/horses/whatever, then that detriment is taken away.
