In every 4X, tech is advanced by a resource usually called "science". The player selects a tech and how many turns it takes depends on how much science they produce. This works well enough, but I feel is uninteresting as there's no interaction between empires, everyone simply advances on seperate, parallel tracks. We also end up with oddities like being in the modern age, but not having fishing tech. Either every tech has to be a prerequisite, or we end up with dead ends like this. I propose a system where *every* tech is being researched slowly. The player can "focus" on a specific tech like traditional 4X (or perhaps multiple, with higher cost or reduced efficiency). When an empire comes into contact another, for every resource/tech they have that you don't, that empire exerts pressure on yours. You lose tax revenue (or growth maybe?) but get some of that resource you don't have and a boost to the automatic advancement rate of that tech. The player now has tough choices, do they continue this trade deficit and have their treasury bleed, do they find/take another resource/tech to equalize by exerting pressure out, steal/research the tech for themselves to reduce/eliminate the pressure, or perhaps attack to either conquer the other empire or force them through a diplomatic "unequal treaty" to give you resources/tech at a "reasonable" price or buy something of yours they don't want. It's important, I think, to have pressure happen automatically so the player can't just turn trade routes off, they're forced to confront this looming economic disaster. The system would emulate the natural spread of resources and technology, as empires in between trade routes (and other empires!) get rich from the coffers of their neighbours having to buy through them. Players can try to blockade/embargo or ban imports and exports, but there will always be indirect pressure from third party empires. Do we close off trade and prevent other empires from "leeching" our tech/resources but we also no longer receive tech "leaks" from other empires and the resources that we need, or open up and deal with it somehow? A lot of 4X games lack "emergent" gameplay, where empires go to war because the players want them to (political), not because the players is "forced" to due to economic/demographic reasons. At any rate, any gameplay mechanic that forces players into choosing between multiple sub-optimal choices make for interesting gameplay as it might be different every time, there's no "best" way to go about it, only less risky, risky and more risky.

While the idea is interesting, I do feel that one of the things that made EL and ES2 special was that you didn't need to research every tech. You just researched the techs with useful results and ignored the ones you felt weren't worth it. This even could be considered realistic to real life; the Inca were a quite advanced society, but never developed a traditional writing system, if they had one at all. Native Americans didn't have horses until they were introduced from Europe, but many of them quickly made use of them afterwards. There was no reason the ancient romans couldn't have built personal crossbows, it just didn't suit their style of warfare, while it did suit the eastern cultures with a similar level of tech. While your point about it being connected to the other cultures you have met and/or are close to helps with these, I feel it's important that if this route is taken that the number of techs that can be "researched" exceed the number that could be researched by even all the cultures together, allowing them to maintain their own identities.

It's not one or the other, the idea of the OP is valid even if the number of tech is so large they can't be all researched.

And I agree, too many techs to be all researched is good.

I feel like having extra techs be leeched from your neighbors would actually work better, so that even if you don't spend the effort to get a certain tech you could still end up with it because your neighbors use it. So say you mostly use infantry and ranged units and intend to ignore riding until a later era, but your neighbors are skilled horsemen, so you end up with cavalry just because of the exchange of ideas through trade.

On the flip side, if you're friendlier with them they'll be more likely to actually answer your questions about what they are doing. "Oh, you want to pet the horse? I'll teach you how to not get your face kicked in."